I'm running KDE 4.2 with compiz. Everything works fine, but when I log in plasma's dock is in the wrong place, the desktop is not shown (except for the part on the bottom where the taskbar should be) and all the windows appear at the top of the screen.
Since a picture is worth a thousand words, here's a screenshot:


The only fix I found is running "killall plasma;plasma"

I'll try disabling compiz and write back.

Okay, now I know it's compiz-related. I disabled compiz and it didn't happen. I'll ask on the compiz forum.