Boy, having AV on servers can be a tricky thing sometimes if those servers have services like Exchange or SharePoint running. If the AV isin't configured correctly to skip files that are in use, it can corrupt entire databases or installations in a flash!
Wile I agree that for admin servers there should be little to no threat since we all know admin's never use servers to browse websites or check email : ), but in the real world I prefer to TNO (Trust No One) and have a good defense in depth strategy in place.
That said, we've had great luck with Trend Micro Office Scan which offer real time scanning and scheduled scans, daily definition updates (sometimes multiple per day) and never has interfered with any critical services.