Um...This happened to me a couple of times already along with those white streaks across screen when moving the camera:
Specs:
Core 2 Duo E6600 2.4 OC'd to 3.2GHz
8GB DDR2 RAM
Radeon 6950 2GB Sapphire DiRT3 Edition, max. temp ~60C when gaming.
Win 7 x64
No problems in any other game, just Dishonored.
Playing it on Steam, haven't touched a thing in any of the .ini files, etc.
Will try the new Catalyst Drivers and see if it helps.