Have flown US Airways from Canada to STT and back five times with a checked PFD and spare cartridges. Always have carried the TSA information on CO2 cartridges as part of a PFD and was only questioned once but have always been permitted to carry as checked baggage. The TSA regulations do clearly indicate it is at the discretion of the carrier to permit or deny. From US Airlines website and others experience, US Airways seems to include them on the non-permitted list as dangerous cargo.
Seems to really depend on the security agent doing the screening.
There is also the option of carrying an unarmed PFD in your luggage and purchase a CO2 cartridge in STT if you have the time to do so.