well, it would be hard to do with 100% certainty, but probably wouldn't be very hard to do with high accuracy.
as i see it there are 3 parts to the issue:
1) identifying which software you have on your machine that you want to check for updates
2) assigning a web page for each such program where the author announces new version
(this is mostly a matter of maintaining some kind of master list matching programs to websites; not hard, but time consuming and could benefit from a community effort).
3) figuring out a way to detect new version announcements on this web page.
(this really shouldn't be too hard to do with fairly good accuracy).