Binary package hint: kde
I had a perfectly stable Dapper install. Last week I updated to Edgy via update-manager, and started having this problem immediately.
Very often, probably close to 90% of the time, KDE will not fully launch. Sometimes it will get stuck at the "Setting up your Desktop" startup step and never display a desktop, requiring a ctrl-alt-backspace. Other times, it will get stuck at the same step, but will bring up a desktop -- but no kicker or Autostart applications. In this case, kicker can be manually launched, but then I can't log out -- ctl-alt-backspace again. Rarely, KDE will start up completely and everything will work.
If I have to ctrl-alt-backspace out of X, on login KDE gives me the error "Could not start ksmserver. Check your installation.", and will refuse to get past the peripheral configuration step. In these cases, it's Gnome or a reboot.
I think i see the same behaviour on my Kubuntu. Occured in Dapper, and now still occurs to Edgy. Altough it's not as often as 90% but more something like 25%.
A workaround is to Ctrl-Alt-Backspace until it agree to starts . It can takes 4 or 5 times. Since it's very undeterministic, i can't debug that, very frustrating.
I use standard Kubuntu Edgy, with skim.