I agree with TheSeven, too. As for MCU, it would be wise to use one that includes USB functionality already. The MSP430F5528IRGCT suggested by Olaf.Mandel seems good, but I don't know the MSP chips well. I have more experience with Atmel AVRs and I would prefer the AT90USB82. That's only a personal preference, so don't give too much weight to that vote, especially if someone else will be writing the firmware.