The ONLY time you need to know Linux is if you enable the development and testing software repositories.

If you stick with the default on the N900 (including enabling Extras) you should be safe from needing to do any hacking.

However if you need to use AOL, Yahoo or MSN for messaging - you will either have to wait until it reaches stable status or take the risk temporarily adding the development repository. That still doesn't require Linux knowledge (in fact that is a negative, its easy for people to do without realising it can screw everything up) just care and attention. Messenger support is safe to install in my opinion, just be sure to then disable the development repository again afterwards if you do so you do not accidentally install anything that IS unstable.