Yes, the iMac has been updated (but no change to the seemingly forgotten little guy in the lineup, the Mac Mini)..
The big change is upgrades to the faster Penryn processors, with 2.4GHz and 2.66GHz on the 20″ model, and 2.8GHz and 3.06GHz on the 24″ version:
Other changes are the increased amount of RAM in the base model (1GB, was 512MB) and improved graphics adapters across the range. As has been the case from Apple recently, definitely evolution rather than revolution.
On a personal note, I’m relieved that my existing order for a 24″ 2.4GHz model is begin transitioned to one of the new models – well done Apple (and a huge sigh of relief from me!)