delete /home/yourcrashedusername/home/kde4 directory or empty this dir.
KDE4 save's the crashed session but debugging or rebuilding is not implemented yet as it seems. I had the same problem after working with firefox and going to sleepmode for 10 minutes. after erasing my tmp and cache files in home/myuser/kde4 it works again fine.
it looks like that kde rc1 works more stabil than rc2 isn't it
I think I'll wait for 4.2 for next look, until then I think I stay with KDE3.5.8.
Good idea of course but stay tuned for kde4, i feel good for the upcoming release
greez
tom