A whole thread on this topic, hell there's a whole message board on this topic: http://www.aircondition.com

With all due respect to Larry, you should have read the link posted by jobah first. The r-406a substitute for R-12 is most assuredly *not* flammable - it contains 4% isobutane, the remainder is comprised of refrigerants just as inert as R-12 or R-134a. The flammable hydrocarbon blends (propane & isobutane) have been banned by the EPA and are no longer available.

That said, one needs to be careful when deciding to switch to an alternative. Most a/c shops only deal with two refrigerants - R-12 & R-134a. The problem is R-12 is too expensive ($37/can at my local mcparts! It was $20 last year) and R-134a doesn't work. Most shops will turn away cars filled with anything else. If you want to use one of these alternatives, it's pretty much DIY.

There are some serious hackers out there rolling their own hydrocarbon blends from camping fuel and such. Apart from the downside risks of hydrocarbon blends, a full charge of the system consumes $0.50 worth of materials. Check out the alterantive refrigerants board at aircondition.com