Viruses are a non-issue from a security standpoint. No one should EVER get a virus, especially not in a business. If you get hit with a virus or worm, you should line up your IT staff and fire pink slips at them, then hire people who actually know how to properly build & manage IT systems.
If security isn't an issue for Macs, then why is there a 50,000 strong mac botnet?
Proper security is an issue for every platform. Security isn't a product, it's a process.