Please all remember that virus protection isn't just about infection, it is also about transmission.

So lets say you are sent an infected document, you open it, read the joke, laugh how funny it is, and send it on to your grandmother.

You did all this on your n900 so you are completely safe. Your grandmother on the other hand is running Windows XP, doesn't know what AV is or windows updates, and trusts you implicitly.

She opens the same document, finds it funny too, but then her PC stops working because you sent her a virus.

I am not saying that the n900 needs av software, but anyone who doesn't consider transmission is part of the problem. This is why clamAV exists - not to stop viruses from infecting linux, but to stop viruses infecting Windows via linux.