I spent a few hours this weekend trying to test KDE 4 in Ubuntu only to find it unusable. I tried to different virtual machines but both had the same problem: immediately after logging in the session ends and returns to the login screen. I spent longer than I'd care to admit trying to resolve the problem but to no avail. I was able to find reports of similar behavior but no useful solutions. I think I may just give up on KDE 4 until the 4.1 version is released and made available.