I think this was the thread you (Patrick) were talking about: http://aspn.activestate.com/ASPN/Mail/Message/pygame-users/2780910
I'm pretty sure this is an SDL thing, I remember a while back with pygame1.7 that I ran into this - I used self-blits for scrolling. I posted about it on here, and it was discovered that it only caused problems when part of the blit was out of bounds. At least I think that was the response... It's not just bliting to self, its blitting to self and something about the rect.