I used BitDefender for several months on my Xandros V2 until I upgraded to V3 and I haven't bothered loading it back on yet:
In all that time it didn't find anything at all and I used to regularly update it as well. I think the main problem is not so much getting a virus yourself, but passing a virus onto someone else via email.
From what I can gather its quite difficult to write a virus which attacks the kernel, most of the virus seem to attack individual applications, which can be easily dealt with by removing the application and reloading it. I checked the virus database and as far as I can tell all the virus which are listed are Windows ones. I think the main intention of Linux AV is to make sure there are none lurking on your machine which could be passed on to some poor unsuspecting Windows user!
I've check the Xandros forums to see if anyone has ever been infected and the answer is no, bear in mind some of those guy's have been using Linux for years, so I suppose they should know.
If you haven't already got one, I would recommend a firewall, I've used FireStarter, its very unobtrusive but does its job well. click here