Came across a good post from Wireless POV that breaks down the number of AT&T subscribers by type (which AT&T doesn't publish in their quarterly financial statements).  It's a guess, but seems pretty educated.

AT&T subscribers (Q4 2008):
60 million postpaid
5.7 million prepaid
11.6 million reseller subs (Tracfone)
= 77 million total

Same methodology should work for other carriers, assuming their resellers have published numbers.