How come this never comes up in any of those discussions about if you go with a Canon all lenses that are not older than you will work with it? Is it just that the 10D is never recommended these days?
Unless you already own a 10d (and should already know the situation) I can't see it ever really being recommended - it is an older outdated camera, and nowadays you should easily be able to get a newer generation for little extra - something like the 30d which is still discontinued, but two generations newer...
So while I wouldn't recommend you go out and buy one, if you already own an older generation camera like the 20d or 30d, you still have a great camera, which will take great pictures, so upgrading is not a necessity, unless there are specific features you are missing compared to a new model.
as for 20d versus 450d, you both gain and lose features swapping to the 450d, higher resolution and live view being the two obvious upgrades, but at the same time losing the better build quality, rear dial, top screen and some of the fancier options of the 20d.
Personally I would be tempted to stick with the 20d for now and put the money in some new glass, what is useful to you depends completely on what you shoot, but you could look into lenses like the 17-40mm, 70-200mm series and some of the higher end primes as great upgrades over a more 'kit' setup.