"I would hate to find in five years that everything I have collected is now pitifully obsolete." so, what should time stop so that this doesn't happen? Your gear is only "obsolete" if it for some reason no longer meets your needs. There will always be better gear and you should be thankful for that. And look either you buy something now or wait until later and buy something then, and you'll be in the same position then as now, with regards to that. as far as the trend to fullframe...yes, it is a trend because aside from SNR considerations highMP sensors start to lose sharpness due to diffraction at lower f#s than lowMP sensors of the same overall sensor size. This is diffraction rolloff. It has to be balanced against the increased resolution and finer noise grain resulting from the higher MP.
Though they can be shot faster also. in the larger sense the DX sensor of the d80 etc is a subframe (aps-c) sized sensor and yes it will work best with DX-optimized lenses, you can put fullframe lenses on a DX camera with a slight loss of performance relative to putting them on a full-fram (35mm) camera. The reverse cannot be done...though in Nikons' case the mount may accept DX lenses and the camera can be shot in "DX compatibility mode" ie using just the center 2/3rds of the sensor. Just like with the D3. I wouldn't worry about the rumored cameras specifically, but definitely this is a trend that is out there.
But that doesn't mean that they can't keep raising the MP of subframes...it means that there is more "headroom" for this in fullframe. You see 12MP p&s's why not 12MP subframes...
New cameras will keep on being released periodically. but not like the September roll-outs of new model year cars. If you have your eye on one now and it does what you want, grab it. There's always another new model rumored to arrive "real soon now." Curent cameras and lenses will continue to work as well as they do when bought. Aps-c lenses aren't going to disappear. Most of the high end lenses are already ff and nothing keeps a ff frame lens from being used on aps-c cameras. The aps-c specific dx, etc., type lenses are usually at the wider range of focal lengths and even if ff cameras supplant the smaller frame cameras, that range would be well and fully populated...
Hm, thought I've responded to this once before. Let me work this backwards... A) yes new cameras will be come out but that doesn't mean that you should either get locked in worrying about that or ignore it. Keep in mind that they may not be all that much better than what is already out now (case in point 450d vs 400d) and they may not actually be any good, or you may not actually want it when you know all that there is to know about it (a700). There's not much harm in trying something that is out now and th en waiting to see what comes out later and upgrading, as long as you 1) buy from a store that will let you return without cost or at low cost within a reasonable period b) don't pay so much for it that you get hosed if you try to sell it on ebay given the newer models. That's another good reason to check on pricegrabber for the lowest price and to rent if possible before buying, if not rent then buy from a store like Adorama or B&H that will let you return it.
That goes for all camera equipment at all times. A store like Best Buy or Circuit City will charge a premium but they will only charge you 15% on returns. Some of the cheaper stores on PG will charge 10% on returns up to one month. I'm not a big fan of renting cameras unless you know specifically what you want to test. At $75-$150 a day renting gets real expensive real fast, though some stores will charge a flat rate for a weekend rental that equals the day rental. But for DSLRs renting cameras and lenses is a great way to go.
2) in general FFs are much better than subframes in terms of noise and that leads to sharper, more detailed images. There's no way around that. There are tradeoffs for ff vs subframe but some things are a given. One of them is that the shots will be cleaner at the same ISO. A 12MP FF will always outperform a 12MP subframe in terms of noise, it will be easier to get the blur down to manageable levels, and the diffraction limit will be higher.
But at 12MP going to 14 is a small change. 6 to 8 is a small change, 8 to 10 is a small change, in the 6-10 region the cameras have about the same SNR, the only difference is in what the mfg is doing on-sensor to reduce noise. Ignoring camera jpegs for the moment. There is no question that newer cameras will have better features and operate faster but there will be little difference in SNR as time goes by. The main problem that is staring them in the face is diffraction limiting.
So given all the above, I wouldn't sweat things too much...products have to come out, you have to buy them, if neither happens then you're not losing much. There's just as much fun in buying older hardware and getting the most out of it and indeed it's a better value than newer cameras as in the end they all do the same thing: take pictures. There's no reason not to buy a camera that is a model or two old and then upgrade a year or two behind the curve. Unless you absolutely need some feature that is on a new camera...and if so, then you've solved the need by buying a new camera. A newer camera with even more features is now unnecessary..
"I would hate to find in five years that everything I have collected is now pitifully obsolete." ...I would hate to find that in 5 years I've paid my yearly salary in taxes, but s*it happens It will not be "obsolete" if it meets your needs. It will not match the features and performance of the latest model, no. But that is the case the moment that the next model comes out. By the way, the new k20 is looking pretty schweet. I'm definitely penciling that in on my upgrade-path. Auto-ISO, anti-shake in the body, 14MP cmos sensor, excellent viewfinder, $1200 retail, gotta love it.
Of course if Canon comes out with a better version of the 5D in the meantime, that will be a true head to head competition. There's no way to deal with a "D10" (a D300 with the D3's 12MP FF sensor) short of waiting until it comes out and seeing what sort of FX lenses are available. You can bet it's still going to be pretty pricey. That's something that I might want to buy in 2 years, but not the month it comes out or anything like that. Even then it would have to keep me from buying a 5D Mk2 or something like that, seeing as how I've already got the perfect lens for that in the Tamron 28-300VC and I already have a 5D...
Ps it will help if you don't think in terms of one-shot solutions or buying a camera that will last a lifetime. Or even 10 years. Also remember that fundamentally cameras are built around sensors, and if you are shooting raw, there's very little that they can add to the camera that will really improve the image. Make the camera easier to shoot, yes. Improve the image, no. Focus on the quality of the sensor, the lens, and the ability of the camera to take images off the sensor without ruining them, and the rest will take care of itself.
It just means that certain types of dust won't be much of a concern. Air guns and dust swabs will still play a role...
Here's a little more info on that D90 rumor http://asia.cnet.com/2008/06/19/nikon-s-mid-range-full-frame-dslr/?scid=rss_c_crv..