;-)

At least some checks should be done.
There are several Antivirus tools for Linux like AVG,
Avast, F-Prot, Clamav, Avira AntiVir (I am not sure, if any of them were ported to Maemo 5).
There are also Web services were you can scan a file before putting it on the N900.
Scanning your N900 binaries in /usr/bin (and similar) from time to time may also tell you if you already got an infection.
Of course you'll never have a 100% security.
Of course the best security software is "Brain 4.0".


There were several threads for this topic:
http://talk.maemo.org/showthread.php?t=31373
http://www.forums.internettablettalk...ad.php?t=61583
and so on...