Further to my previous comments, it appears that the scan line/graphical corruption was not due to the 11.11 drivers, but rather, I had somehow managed to switch off my custom fan profile in Afterburner. So the card was getting a little bit too hot, what with the overclock!
I turned it back on, saved the profile, and after hours of play, I've not had it since!
I did still roll back to 11.10 drivers though as they worked fine, and I did notice some fps drops withn the new one, where there werent any before.
As an aside, is anybody running a single 6870 in eyefinity on BF3, if so, what sort of framerate are you seeing? I'm running mostly high settings, 16x AF, no AA and 3840x1024 resolution - seeing late 40s to mid 50s fps, which I think is pretty respectable. Got a score of 699 on unigine with this setup!