Just to be clear - on a minor issue. Some airlines will provide oxygen canisters, for a price. So, folks who need these canisters are able to travel by air.
Just to be clear:
Southwest does not allow compressed or liquid medical oxygen. Southwest does not provide oxygen canisters at any price. Their webpage with this information has contact information for companies that provide portable oxygen concentrators. This and more information at
www.southwest.com/travel_center/medications.html
On Delta, onboard medical oxygen may only be provided by portable oxygen concentrators. These can be either owned by the passenger or rented from outside companies. Delta does not provide oxygen canisters at any price. This and more information at delta.com/planning_reservations/medical_concerns/index.jsp#oxygen (may not be entirely accurate, trying to read it off the iTouch)
AirTran will refuse to transport a customer requiring... medical oxygen for use on board the aircraft. Portable oxygen concentrators are allowed, but while there's a list of approved devices, there's no source or contact information, or any indication that the airline might even possibly provide POCs. More information at
www.airtran.com/special_needs.information.aspx#poc
Most in-depth information found at JetBlue.com. "JetBlue will not carry persons who require medical oxygen onboard. Due to safety reasons, [they] do not permit any type of oxygen to be brought on the aircraft nor do [they] supply medical oxygen onboard... Specific models of oxygen concentrators are approved for onboard use... An oxygen concentrator is not to be confused with compressed oxygen which is prohibited on any JetBlue aircraft."
Very explicit explanation of procedures readily available by visiting JetBlue.com, then selecting Help, then typing
oxygen into the search box. Again, though, nothing indicating they provide the portable oxygen concentrators at any price.
Continental's site was enlightening. "Generally the ...
FAA prohibits the use of personal oxygen units during flight because they contain
compressed gas or liquid oxygen which are defined as
hazardous material. However, the FAA has recently issued guidelines permitting the use of certain portable oxygen concentrators" (all emphasis mine). Also, "Continental does not provice onboard medical oxygen. Customers may carry onboard and use FAA approved personal oxygen concentrators", and then the site provides a link to its Customer Provided Oxygen Concentrators page.
American Airlines assesses a $100 charge per flight segment

for oxygen provided by its contracted (I guess) company.
United allows compressed oxygen canisters in the cabin or in cargo provided the cylinder is empty and stored in a cardboard box, but doesn't allow liquid oxygen period. They, too, have an oxygen provider but they're bigger thieves than AA - they charge $125!
US Airways provides information about traveling with portable oxygen concentrators and al link to a company that rents them.