I really do think a lot of distros will keep 3.x for as long as they can.
IMO, KDE4 mocks vista too much. The drastic changes in the way the menu works and how it's designed, The desktop management is done differently, it all just seems to vista-like to me. And worst of all, still as buggy as it was before it went final.
I haven't been a KDE fan for a long time... and it'll be a while before I switch to KDE4