I have the same problem with yawp, the mail plasmoid and, especially annoying, stasks.
For the last two weren’t updated for some time, I think the error is somewhere else, perhaps in the last Qt update?
I know, if something happens, it’ll be my fault (“Don’t use betas for work!”), but I solved the problem by installing KDE 4.3, which since longer time works like a charm on my other machine (which I normally use for trying things out).
Perhaps today’s update to 4.2.4 can help?