Viruses and root exploits are two different things.
If you give a typical consumer a windows CD and a Linux CD and have them install both, neither system will be secure. An OS is only as strong as it's weakest link, and that is always the user. There are regular root exploits of linux announced on CERT and other security lists. All the user needs to do is keep on top of the hotfixes and updates. The average user won't.
OpenBSD is our office firewall. One root exploit in 10 years in the out-of-the box release is a hell of a good record.
We got a new dual-G5 at work today for development. It's a sweet machine. If Macs weren't so overpriced I'd get one. The stability of a BSD based OS, combined with the Apple's interface design isn't a bad thing. Although my intense love of gutting my hardware regularly to upgrade or tweak would have to find a new outlet