It looks like this commit added the 2 pixels : http://websvn.kde.org/branches/trinity/3.5.13_frozen/kdebase/kicker/taskbar/...
I guess it's something related to the QT4 port of TQT. You can try to revert all the "-2" in the code and recompile to see if it fixes your issue.
Now, how did you find that?!
Sure, I'll see what happens.
Darrell