Has anyone ever encountered trails/artifacts as in the picture while playing SC2?

I've tried other 3D apps on the machine such as Portal2 -- That worked just fine. Wondering if anyone has encountered this before and if so how it was resolved.
Additional info:
Hardware Overview:
Model Name: iMac
Model Identifier: iMac11,3
Processor Name: Intel Core i5
Processor Speed: 2.8 GHz
Number Of Processors: 1
Total Number Of Cores: 4
L2 Cache (per core): 256 KB
L3 Cache: 8 MB
Memory: 4 GB
Processor Interconnect Speed: 4.8 GT/s
Boot ROM Version: IM112.0057.B00
SMC Version (system): 1.59f2
ATI Radeon HD 5750:
Chipset Model: ATI Radeon HD 5750
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 1024 MB
Vendor: ATI (0x1002)
Device ID: 0x68a1
Revision ID: 0x0000
ROM Revision: 113-B9710C-238
EFI Driver Version: 01.00.417
Displays:
iMac:
Resolution: 2560 x 1440
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Built-In: Yes
Connection Type: DisplayPort
Display Connector:
Status: No Display Connected

Status: No Display Connected?? – Bora Jun 6 '11 at 23:36