Students often ask about federal regulations governing inflatable vest cartidges in carry-on and checked luggage. The regulations have changed several times over the last few years. As of today, this is the current regulation:
Federal Regulation 49 CFR 175.10 (11)
(11) A self-inflating life jacket fitted with no more than two small gas cartridges (containing no hazardous material other than a Div. 2.2 gas) for inflation purposes plus no more than two spare cartridges. The lifejacket and spare cartridges may be carried in carry-on or checked baggage, with the approval of the aircraft operator.
The key phrase is “with the approval of the aircraft operator”. It is each airlines choice whether to allow these on their airplanes, and each sets their own policies (currently about 1/2 allow them and 1/2 don't). We recommend checking with your airline (and returning and connecting flights) in advance. You can see the regulation here: [Federal Regulation 49 CFR 175.10 (11)]. Also, look here under "Disabling Chemicals & Other Dangerous Items" for specifics: http://www.tsa.gov/travelers/airtravel/prohibited/permitted-prohibited-items.shtm
Even if your airline permits them, you may be prepared with copies of the Federal Regulation and the regulations for your specific airline, since it's not a common item and airline personnel may not know that they are allowed.