[font color=\'#000000\']Since I believe that no-one has a 20D yet except reviewers, let me pass on what I have read.
a) as with all but a few high end dSLR's, the auto-focus system requires lenses of f/5.6 or faster. How badly the performance falls of beyond that, I do not know. (I believe that so many modern zoom lenses are f/5.6 at the telephot end, and so few are slower, because of this widespread requirement of AF systems.)
the center AF point is cross style, but reportedly only has that full functionality with lenses f/2.8 or faster (e.g. not with any EF-S lens).[/font]