I too can't wait have a go with MAME on a Raspi. A few years ago I built a 2 player arcade controller using proper arcade buttons and joysticks, some MDF and a USB keyboard. It worked ok but the USB keyboard interface wasn't geared up for multiple simultaneous key presses and so there were odd quirks. e.g. Player 1 moving up/right could user fire button 1.
Fortunately there is a proper solution to this in the name of I-PAC from Ultimarc.
This has been specifically built for use with MAME and other emulators and can cope with everything being pressed at once. Best of all it pretends to be either a PS/2 or USB keyboard and so should just plug into the Raspi without any hacking required.
Ironically it costs about the same as a Raspi but rather than that being a ripoff it only goes to show how damn good a price the Raspi is.
I have my I-PAC ready to go, I just need my Raspi now so I can convert my controller into being something that can plug directly into any TV rather than needing a computer.