Professional virus writers often target only small group of machines. They sell bot networks, and these are more likely not to be noticed by anti-virus software developers when virus is not spread wide. It would be possible to use N900 for e.g. sending spam even though wireless mobile device hasn't got the best performance. Also there's no anti-virus software for N900 and root access is not protected.

Though i'm sure that this forum would notice a virus right away.