Just to provide a little bit more clarification, RIL stands for Radio Interface Layer. It is essentially a driver for the radio that allows the OS to talk to the radio. Because of how altered the Sense skin is on top of Android, the RIL for sense based ROMs is drastically different from that of AOSP ROMs. This means that in order to communicate with the radio with an AOSP ROM, the RIL has to be completely rewritten by hand. Slayher is hard at work on this, and its more or less the only thing holding CM7 for the TB back. Once the RIL is complete the CM7 for TB code will be merged with the mainline, and the ROM should be almost at a stable point for the TB.
Like Jax said, feel free to work on an AOSP ROM for the TB (and unfortunately I can't provide you with any information on where to start with that), but don't expect to be able put out a feature complete ROM until Slayher finishes the AOSP RIL. Unless, of course, you're very good at writing low level code and beat him to the punch =D. In which case I'm sure he'd be most appreciative lol.