Could be the graphics driver I suppose. I'm using NVidia.
I tested on my laptop, and while it doesn't have exactly the same problem, the location of the menu bar changes slightly as PM starts up.
On my desktop I can duplicate this "laptop" behavior by starting PM and then resize the window. As the window is resized, the menu bar drops down a few pixels.
Interestingly, if I hold down the shift key when starting PM on my desktop, I don't get the problem. But without holding down the shift key, the problem persists.
Deven