After much research on this same topic, I have yet to come up with anything. I know with other Moto products, they like using the combo of vol down + power, or sometimes they mix it up as vol up + power, or even more rarely vol up + vol down + power, but that doesn't happen too often. After trying all these key combos, none seemed to mean anything to the ATRIX. There may be something with the soft keys (menu, home, back, search) but those generally stop responding if the phone is in a loop, so I would be suprised if Moto did anything with those.
This does not mean there is absolutely no key combo available, it just means I haven't found anything, so if somebody knows of a combo, it would be swell if you share.