There's not yet anyone much programming malware for Maemo. Once upon a time there were no need for MS DOS antivirus either. But thinking security on ANY platform is good practice, even if there's no known exploits - yet.

Virus is an outdated term anyway, I would believe a rootkit is a more reasonable threat to worry about on this micro-webserver known as N900.

Edit: and of course, transmitting viruses. If you first hook this up as a drive on a virus infected machine, and then on another machine, the virus may be transferred.