Thanks to a reader who has been doing pre-trip research, I recently learned that
to visit Calakmul, you must take a shuttle bus from near the biosphere entrance on the highway down to the ruins themselves (about 40km). There’s also a new museum there, where you catch the shuttle. Apparently there’s also some kind of fee for the shuttle.
It sounds like a hassle, but frankly, it’s good. Driving that road down to the ruins is really a drag–very narrow, and always filled with birds and beasties. (This is apparently the reason they instituted the shuttle–too much roadkill.) So better to leave the driving to someone else.
I’m headed down that way soon for research on the Rough Guide to Mexico, so will check this out in person and post details here.
EDITED 10/26/12: The shuttle bus is no longer in action (apparently because the local taxi drivers couldn’t come to an agreement with INAH over a fair way to do this). You can drive your own car in, or go in whatever taxi you hire. So basically, it’s the same as it always was. (The “museum” is pretty nominal.) Admission prices as of August were M$46 for the ruins, plus M$28 per person municipal fee and M$54 per person for the reserve.