A virus is all about target audience. You want maximum hit to get what you are after, be it money or just general damage to the system. Maemo is such a niche market and such a small and shrinking market there really is little impact in creating a virus for it.

As an OS Linux is more stable and harder to break than Windows because of the nature of the kernel, and the fact the structure of drives is different, then each linux distro is different so viruses have a hard time jumping across and then you factor in the mobile aspect again has differing names for things and then factor in that you need ssh (or something similar) downloaded to your N900 to even do anything remotely like affect the phones core files, all of this means it is in no ones interest to create a virus for the N900 in the first place.

Not saying you can't get one, just saying no self respecting virus writer would try. It is not like they can skim money off you while you browse the app store, it is so dire you won't spend money at the OVI store so they see no returns for their effort. So what would they steal that Nokia hasn't already with the 1.2 data grab? Easier to hit Apple and Android phones, where the people are.

Of course you might get some really hacked off developer that does it to make nokia sit up and to give them some bad press.

That would be a shame wouldn't it.