Rootsh is available from extras AFAIK, no need to dive into devel right now. Please stay out of devel as long as it takes to feel really comfortable with installing and removing things from your N900.

Most applications are already in Extras. Even testing is in my opinion pretty safe to use, and you can do the community a favor by voting for apps that work as intended.

http://wiki.maemo.org/Extras-testing...lity_Assurance