Thanks for making this post, it's hit on a note that I knew but never realized (if that makes any sense...). I notice this when I compare images from my 350D to my 1Dmk3, it's hardly a case of comparing apples to oranges. In fact the difference, or lack of, is very comforting as it allows me to not bother with technology concerns and simply go out and shoot. The 350D is where I believe DSLRs really made the greatest advancement, allowing an affordable performance leap unseen before and since in 35mm digital.
Another method of judging technological advancement is by looking at an industry's flagship model. The 1D has seen a lot of changes, however the change from the MkI to the MkII was huge. That's very contrary to the MkII to MkIII. The main remaining improvements are in the realm of ISO, pricing and sensor size. Aspects like AF, ergonomics, and colour rendition are all about as good as it can get from a 35mm. The only REAL noticable improvement in the next years would be an increase in sensor size I'd imagine. Hopefully they'll also be able to stick these larger sensors into smaller bodies.
Expect future 35mm digital to only get cheaper and not tremendously better, at the moment the real advancements in digital photography are in medium and in a few years large format.
