See above: 200MB install for 3.5 (less after the Service Pack comes out EDIT: didn't know how small it was, see below) (22 for 2.0, 50 for 3.0).
2.0 is installed already on all XP SP2 computers (and maybe SP1 too, don't remember), 3.0 on all Vista computers.
Sure, if I was writing the program for myself only, or for .Net programmers only, I'd be using the latest version without any hesitation;
but I'm not.