Symantec: Mac OS X Becoming a Malware Target
Regardless of the merits or motives of Symantec's prognostications, as I have told my clients, just because there are no viruses or malware in Mac OS X, it doesn't mean that there won't be a really nasty one released in the next five minutes. Better safe than sorry.
The other reason to have anti-virus software on Mac OS X, is to prevent those Mac OS X workstations from unwittingly passing on PC/Windows centric viruses. This is especially important in multi-platform environments.