I've read many threads discussing the 'danger' of hydrocarbon (HC) refrigerants. This has been discussed to death a number of times on the popular automotive a/c web sites:
aircondition.com
ackits.com
I invite people new to the subject of HC refrigerants to spend some time on these sites.
Everybody points to the worst-case scenarios, but looking at this on a practical level, there are many cars running around with HC refrigerants (mainly in Australia), and a terrble incident has yet to happen that would support this fear. It is simply very difficult to put yourself or your car in a position where the HC refrigerant is an explosion hazard. By fact that we are talking about automotive use, it is difficult for HC refrigerants to leak out and concentrate to the level that it is an explosion hazard.
As Gary pointed out, these nay-sayers often ignore the fact that most cars are running around with 10-20 gallons of very volatile and flammable fuel, which has a higher danger and risk to the driving public. I just can't get excited over 8 oz of HC refrigerant, which is the typical fill for automotive a/c. HC refrigerants also use a lower head pressure than r134 or even r12. There is just something in our American culture that is resistant to HC refrigerants.
If you check the r134 MSDS sheets, you'll find that it is flammable.
Cryptokid, you mentioned that you converted to propane. Was this 100% propane? The popularly available HC refrigerants are chiefly a propane/butane blend. I forget the exact issue, but there's something about pure propane that makes it undesireable for a/c use.