FYI here is the updated Canada guide through the TSB. If you find the applicable US list print it out and take it with you..that helped me once in Miami on Air Canada.
http://www.catsa-acsta.gc.ca/english...age/list.shtml
It clearly states as an exception under sporting goods:
Carbon dioxide (CO2) cylinders or cartridges (except those required for medical purposes; those to be used for aircraft maintenance or for aerodrome maintenance or construction; or those used for self-inflating life-jackets).
No No
CO2 cylinders for self-inflating life-jacket
Note: No more than two small CO2 cylinders or two cylinders of other suitable non-inflammable, non-poisonous gas when fitted into a self-inflating life-jacket for inflation purposes, plus no more than two spare cartridges per passenger.