Very strange, not seen that before, maybe a dodgy install. Or a graphics problem what are your system specs and temperatures. Also is it a steam version or DVD.
Intel core 2 Quad Q9550 @3.0Ghz
BFG Tech GTX 285 OC
Asus P5N-D Mobo
4GB DDR2 Corsair XMS
Thermaltake Ruby Orb Cpu cooler
EZ COOL 650w PSU
Put "mip-map detail level" to the highest setting in your graphic's cards settings and make sure "Alternate pixel centers"(if using Ati tray tools) is unchecked.
Fincuan's absolutely right.
This only happens for me if I set Catalyst to 'custom' and then put mipmap settings to the very bottom position.