I have an old Phone-as noted in the title--the Atrix 4G and the HD Dock (don't laugh, got the dock on clearance.) It's a good little phone. I am considering turning it into a mame machine. With the dock, it has USB and HDMI out with sound and power. In dock mode, I can use Linux or Android (mame4Droid confimed to work, even with the older android).
I also have an Xarcade dual stick (so since it is a keyboard, it should work). I've seen Raspberry Pi's modded into this and was thinking of doing the same with this phone.
Phone and hd-dock (avoid the standard dock) can be bought on e-mail for about $35 dollars. May be cheaper that the PI (especially if you already have one, like me).
Curious if anyone has tried this yet?