I took a big gulp and bought a Nikon F3 in 1990 and swore that this would be my last camera. Well, it was my last 35mm camera until I went digital two years ago. That was 16 years, which is a good run. My first digital SLR--the 30D--is now a backup body after two years. Technical improvements are so fast today that I think you should expect only a few years of state-of-the-art service from a camera. That doesn't mean that they stop working, but they will be easily surpassed in every dimension. I'm not an industry guru, but my guess is that soon enough we will have sensors with "variable resolution" and "video shutters" that make a lot of today's discussion seem antiquated. The 50D has "gapless" sensors, so in principle it should be able to offer a range of resolution/noise levels without loss. I feel certain that this is coming. The 5D2 has 30FPS video at HDTV resolution. Why not 100FPS at 640x480? or 1000FPS at 320x340? Surely this is coming, too. Anyway, I think it's hopeless to buy a "future-proof" camera today. I would recommend the 50D over the 40D primarily because it offers the lens focus micro-adjustment, which I think is a critical feature to get excellent results. I would not get a 1D body unless I needed its features for a specific reason just because its too heavy, bulky and conspicuous for travel. I would rent a 1D body if I had to have one for an event. I would recommend the 5D2 over the 50D if I wanted to use fast/specialized primes vs. high-quality EF-S zooms because those prime lenses don't deliver their full value on a crop body. I would recommend the 5D2 over the 5D for focus micro-adjustment and live view, which I think are both critical features. I would hate to give those up for an older 1D body. So far, I'm finding that the 9-point AF on the 5D2 works very well, but I'm sure that the 1D focus would be even better. I was worried that the focus points would be too close together, but they are fine. The main thing I find lacking in the 5D2 is manual control of aperture when shooting video. I'm sure that will be addressed in the 5D3 and the 60D!