Nope- it is not possible. At least not yet. They are still working on cracking the bootloader for the atrix- so far it hasn't been done. So we havent been able to do any true custom roms.
Pretty much all that can be done, so far, is theming on our current OS (froyo 2.2) and little things like, deodexing and stuff like that...
This is at least what I understand, I am no android expert though.
when it comes to learning exactly what all those things are- deodexing, SBF, sideloading... all that, I personally don't know a page that addresses all of that stuff... I just kind of learned over time from google.
Sorry I dont have any better answer for you. :-/