I will have to dissapoint you because you are very wrong my friend. The apps installed from repositories can have root access to everything on N900 without you even knowing it (you will not be asked to give the app any permission - it can transfer all your personal data at ease). This means that N900 has no security at all. And belive me it is very easy to upload something to extras-devel. I think many users have extras-devel enabled (are there any statistics?) because its the single place where most N900 software is available.

Also Maemo 5 will propably not get much updates. I hardly belive security issues would be addressed in update at all. That would need a huge redesign of whole OS.

Android and iPhone have some level of security built into them and once a security hole is discovered there are chances users will be provided with a fix. Thats totally opposite for N900.