First off, has anyone stopped to consider that Jobs might just be trying to get his friends at Symantec, etc a little larger holiday bonus? It wasn't made quite public enough for that to be probable, but it is still possible.
Second, to whomever thinks that they shouldn't find and erase Windows viruses from their Macs... consider that sharing viruses is considered quite impolite, and could be construed as illegal, though if you didn't know it was there you'd probably be alright. Still, it isn't cool to put others at risk because you are lazy.
As others have mentioned, ClamAV isn't exactly optimal for desktop use. It was / is designed mainly for servers, where it gets called to scan files / volumes on demand, like checking an email attachment before allowing the message through, or daily scans on user shares. It does have real time scanning support in Linux, but it doesn't seem terribly mature from what I've seen. It definitely isn't a replacement for realtime scanning, which is what you need, if you need a virus scanner at all.
Unless Norton has improved a lot since version 10.0, it's crap. And uninstalling it is nearly impossible. That app caused me so many problems with my previous job at a helpdesk that it still makes me fume to consider how poorly designed it is. And it does tend to slaughter your performance.
Some people seem to make a big deal about *nix security and what not. A computer is secure as you make it, and not all *nix systems are equally secure. For instance, as far as I know there is no SELinux analog for OSX. Then again, it's easy to give every account root privledges and no passwords on Linux.
The main problem with realtime scanners is that they use kernel hooks that get called frequently. With 2 or more realtime scanners, you interrupt the kernel more frequently, and run the risk of having the (generally sketchy) kernel hooks interfere with eachother.
I personally don't like AV on Windows, because #1) I can remove viruses better by hand, #2) I keep an eye on processes and whatnot and I generally notice a virus quickly, and #3) I definitely notice the performance impact on heavy file IO.