I got this working almost fine with 2.12 driver, but with 2.15 I have to switch to xrender. Using openGL results in widgets flickering. The system becomes very hard to use. Same results with Natty, and this stops me from upgrading.
I tried Ubuntu 11.04 today, and enabling the hardware driver results in unusable system - big black rectangles appear where the mouse should be, so witch KDE it is much better
I run Thinkpad x40 with 82852/855GM video.
Just wanted to share, I

This sounds like it could be a defect in the Intel drivers. Have you tried reporting a bug against the Intel driver on the freedesktop.org bugzilla instance?

See my related post:

viewtopic.php?f=111&t=97085&p=204553&hilit=kwin#p204553.

I don't have a solution yet, but it's good to know the problem is not exclusive to one user.

(selecting XRender)
It solved my issue. Thanks.