My current system is a Core Duo Mac Mini and I'm using it as a test bed for when I finally get the cash together to build a cab. It runs MAME and all the 16bit emus really well, but frontends are a bit thin on the ground.
EmuLaunch looks nice but is lacking in features and has been discontinued. It works well though. (
http://www.emulaunch.com/)
Supposedly Wah!Cade can be compiled for OS X but I never got it to work. (
http://www.anti-particle.com/wahcade.shtml)
IIRC SDL MAME has a game list function built in, so if you're going purely MAME you might be able to configure it to load on boot.
However, none of those solutions panned out for me so I now dual boot Windows XP and run HyperSpin. Using Windows also allows me to play N64 and PSX games.
I'd definitely consider using my Mini in a cab but not running OS X. However, I think if you're going to go out and buy one specifically for a cab, you'd probably be better off with a cheap PC.
Good luck with your project, I'd be interested to see what you come up with.
