MeeGo 1.2 and N900 Developer Edition are 2 different things. First one is the OS release, and second one is an edition optimized for N900.

At any time you will find everything there is to know at the wiki: http://wiki.meego.com/ARM/N900/DeveloperEdition. Most important thing to understand: This is not for regular Maemo 5 end users, only developers and hackers.