Aperture is Macintosh only at this point & I would bet it would stay that way -- none of their other Pro apps are cross-platform.
I would guess that Lightroom uses some newer DirectX versions for its video tricks that may not be supported in Win2K. Plus, with Vista coming (eventually) Adobe doesn't want to have to write to 3 different OS revs.
Does anybody know why Lightroom won't even install on Win2k? How about Aperture, is it usable on Win2k?