The current plan is for game logic to either run on a Chumby Hacker Board, or the Chumby based Insignia Infocast board. The Chumby and its derivatives are very hacker friendly - the hardware is designed by Andrew "Bunnie" Huang, author of Hacking the XBox.
The P-ROC (Pinball - Remote Operations Controller) is a circuit board that enables a computer to control a pinball machine over USB. It is an extremely clever item built by Gerry Stellenberg at Pinball Controllers. Not only does the P-ROC work for a completely custom machine like Frank, but it can actually replace the CPU board in a standard Williams or Stern machine, allowing you to develop your own custom ruleset for an existing machine.
"Driver 8" is a board that I designed myself and had fabricated by Seeed Studio, using their Propaganda PCB Service. It is basically a slightly beefed up version of the auxillary driver board found in some Williams machines, with the addition of onboard fuses. If anyone is considering building their own version I recommend separating the power connector and fuse holder slightly more than the current design, which works but is a very tight fit. (I made extra boards, so if you just want one to play with contact me, I may still have spares.) Because the P-ROC makes it so easy to use a real Williams driver board these won't be used in the final Frank machine.