I've been spending quite some time lately to re-implement the host-side driver for the Omnikey CardMan 5121 RFID part.
The 5121 is an Atmel AT89C5122 based USB CCID reader, extended by a Philips CL RC632 RFID reader ASIC. The RC632 is a quite common reader ASIC found in many ISO 14443 A/B and ISO 15693 readers of today.
Since the RC632 is common, I'm actually writing a generic RC632 driver. Below the driver there is a "transport layer" that is specific to the CardMan 5121. It's my hope that over time it will be possible to support other readers by adding device-specific transport layers. Above the RC632 driver, there are implementations of the ISO14443 A and B anti-collision algorithms, as well as some code specific to the I*CODE and Mifare proprietary transponders.
So far I think I've written about 60% of what's required to access my MaskTech MTCOS 1.1 (14443A) card.
One of my major obstacles was not related to the RFID stuff at all. I've never learned as much about ELF PIC (position independent code) in its x86 incarnation. For some reason IDA Pro's code analyzer doesn't fully recognize the PIC format of the proprietary driver. It always works for any other .so file I open, but not for ifdokrfid.so from Omnikey. Maybe they're using some strange compiler (or compiler options)...
I'm confident that within the next couple of days I'll have the system running.
As an interesting side note, the RC632 seems to be able to just passively receive, too. I didn't have a chance to confirm this yet, but looking at the docs I have, it should be possible to demodulate/decode without actually sending the main carrier or any commands to the PICC / tag. I always thought that vendors would build their chipsets in a way that no easy eavesdropping was possible. Well, we'll see.