Not that this means the TSA agents you run into will know their own policy, but here's a quote straight from the TSA site:

"Flameless Ration Heater (FRH) - A product most commonly found in MRE's, used to heat food packages. There is no limit on the number of MREs an individual may possess in checked or carry-on baggage, but the MRE(s) containing FRH(s) must be unopened (original factory seal is unbroken)"

http://www.tsa.gov/travelers/airtrav...ted-items.shtm

I'm going camping in two weeks and will bring MREs. I will print out this page from the TSA site, but that will likely be no match for incompetence.