I had the same problem and in my case it was caused by incomplete entries in the Qt configuration file:
Particularly by the entry which is apparently related to the active KDE theme (Oxygen in my case):
usr\lib\kde4\plugins\styles\oxygen.so=0, 1, , 2012-08-07T19:49:32
found in a section named:
The fix is to remove this entry and restart Skype. After that the entry should be re-added to the file, but this time it should be complete and Skype should use the same theme as the rest of the desktop. The re-added (complete) entry should look something like this:
usr\lib\kde4\plugins\styles\oxygen.so=40802, 0, i386 linux g++-4 full-config, 2012-08-07T19:49:32
You may want to remove other incomplete entries from the file too before restarting the Skype, but in my case it was enough to remove just the single entry to make it work.
Note that it seems that the incomplete entries are created by any Qt program for all libraries it stumbles across which are of a different architecture than the program itself (if the entires for the libraries are not already present). I.e. if a 64bit Qt program is run it creates incomplete entries for all 32bit libraries it stumbles across (if the entries for them are not already present) and vice versa - a 32bit Qt program creates incomplete entries for all 64bit libraries it stumbles across (if the entries for them are not already present).