People have explained that open source computer environments are much less prone to these things than close source ones. Just look at the number of viruses, trojans, keyloggers etc. for Windows (hundreds of thousands), compared with the number for Linux (very few, but not zero).

That said, I have seen two compromised Linux servers in my career; it does happen.

But the smartphone world is a little different.

Unlike a laptop or desktop, closed source smartphones are quite restrictive about what you can install. So you're not as likely to install malicious software on a closed source smartphone, compared with a Windows desktop, simply because you aren't allowed to: the only things you can install are "approved".

Whereas on Maemo, you have freedom to install any old junk, and the temptation is surely there to install things you haven't compiled yourself...

We rely on the community to check things, and for the most part, it does. We also rely on distributions, in this case Maemo and Maemo-extras, to check things and often to ensure the source matches the binary. Amd, when something is found out, if you are updating regularly, there's a good chance it will be fixed quickly.

The same applies to closed source: with their app-approval processes, that provides a similar kind of checking.

But a major difference has to be on Maemo you can install anything, from anywhere, if you are stupid or if you are tricked into it. With closed source smartphones, that's harder.

It has been said that Linux is inherently more secure than Windows, by design. But it's also been said that Windows has so many malicious programs because of user culture / knowledge / security practices, and simply because it's the more popular platform so it attracts malicious software writers, which combined with the ease of cracking it, tips the balance strongly in its favour.

N900 looks quite a tempting target, if it gets a huge amount of users.
But it is developed by people who are quite security conscious, and a community which is also conscious of such things.

So it remains to be seen which smartphone gets the first virus making premium-rate calls in the background...