KDE 4, code-named Knut
I personally prefer Gnome over KDE but with this new and polished look available on version KDE 4, code-named Knut, it may just be enough to make me switch. The new look-and-feel is beginning to be more Mac OS X-like, imho. Dolphin, the new file manager looks more intuitive, though. 
Whilst still in its alpha stage, KDE 4 is expected to be released some time October 2007.