All versions of KDE 4 (including latest 4.2.0) on my system (Gentoo Linux 64-bit) seem to have this bug when rendering fonts in Plasma areas:
(Note the rendering bug in the words "Free" and "Year".) I'm using version 2.28 of the DejaVu fonts. The Bitstream Vera Sans font does not have this bug. Unfortunately, I don't know if the problem is with KDE, Qt, X11 or DejaVu. But since Vera Sans (and all other fonts I tried) look fine, I suspect it's a DejaVu bug.
Created attachment 22555 [details]
Screenshot of the problem
In case you have problems with the tinypic URL, I'm also attaching the screenshot here.
I forgot to mention something I observed. This bug only shows up in certain letter combinations only. So far, I noticed "Fi", "fi", "fl", and "Ye". There may be others I didn't notice yet.
I'm closing this again. It turns out the problem also appears with other fonts. I apologize for the noise.
Nevertheless I'm interested in the follow-up of this (so we may have an idea where to point to if someone else comes to us with this problem). Did you enter this in the KDE bugzilla somewhere?
I just submitted one.