Other than the previous poster's anecdotal evidence, I didn't see anything on the TSA's or FAA's websites that would prevent the traveler from bringing a heating blanket past security and onto the aircraft. However, actually plugging it in would require the approval of the PIC or airline (reference) due to the fact that it's "heat producing equipment." Since these blankets have a penchant for spontaneously catching fire, I'm not optimistic that the crew would be okay with that.