I got my N900 end of jan,. I am a linux noob .... started out using just the extras testing repo as from what i can read the worst that can happen in testing is an app could cause issues but uninstalling the app will sort it out.
then i started looking into devel repo and installing stuff from there .... i know that an app may mess my phone up totally but by reflashing i can sort it out.
I am now starting to (tentativley) use xterminal to change stuff (added the reboot button to power key menu)

All scary stuff but you gotta live on the wild side eh!