Thanks to the Ubuntu forums, I’ve finally managed to fix the broken osd / notification system in 9.10 on a t41 thinkpad (the bug affects all radeon based devices, I believe) by adding the line
Option "RenderAccel" "off"
However I had to create xorg.conf first, check the Ubuntu forums for more information (search for “karmic no xorg.conf”), for example. It was also suggested to limit / set the AGPSize to “32”, but sadly this did not solve my problem.
On the upside, “Stellarium” runs again, on the downside, it’s now slow as molasses, but that’s still better than a segfault in my book. Why the developers broke something in karmic that worked without a hitch in jaunty and earlier is beyond me, however.