Adding my two cents worth:
At this time there have been no confirmed Mac OS X viruses (subject to the semantic debate about whether the iChat exploit is properly classified as a virus or a trojan horse and not counting the Word macro virus that can affect Office v.X or non-updated copies of Office 2004), very few trojans and no adware or spyware. I therefore do not feel that antivirus or antispyware software is necessary at this time as long as users are careful about what they download from the Internet or accept as attachments via email.
It's never wise to become complacent, though, so security precautions are not wasted. Such security precautions mostly are common sense: don't download and run files from sites you don't know, don't double-click on attachments in emails you aren't expecting and/or from people you don't know, and make sure you password your system and keep that password secure, In addition, don't leave your your system available in a public place and unattended where someone unauthorized could use it and potentially install malware (such a policy is also a precaution against theft).
It is possible for a Mac user to inadvertently forward a Windows virus to a PC user though you'd have to do it manually so the above precautions would probably prevent such virus-spreading, and said Windows virus cannot infect the Mac.
If you do decide you wish to run antivirus software, I think that the donationware
clamXav should be more than enough provided you remember to scan your system from time to time. iAntivirus has also been recommended by others, though I have no personal experience with it.
If you will be running Windows on your system, either via BootCamp, Parallels Workstation or another solution, then that copy of Windows is subject to all the myriad exploits common to the Windows world, so you need to take full precautions, including running both antivirus and antispyware software.
Regards.