propane, butane, and isobutane, used a propellant...?

Yes, those are all greenhouse gasses, and releasing them into the atmosphere contributes to GW.

The higher molecular weight hydrocarbons are oxidized rapidly in the air, and are not a serious problem, in the relatively small amounts used as propellants, either for smog or AGW.

"The alkanes have estimated atmospheric lifetimes of 8 years for methane, 2 months for ethane, 2 weeks for propane"

http://www.unca.edu/chem/squibb/2004.htm

The problem with chlorofluorocarbons as propellants or refrigerants was strongly related to the fact that they are extremely stable (people thought that was great when they were first used as refrigerants) and so build up over time. So this time they deliberately looked for a non persistant alternative for propellants.

EDIT - 3DM raises an interesting point. But a search showed that he might not be right about the oxidation products of propane (and maybe the others).

http://www.agu.org/pubs/crossref/2007/2006JD007566...

This hardly seems definitive, though.

Bob said: "The higher molecular weight hydrocarbons are oxidized rapidly in the air, and are not a serious problem, in the relatively small amounts used as propellants, either for smog or AGW...The alkanes have estimated atmospheric lifetimes of 8 years for methane, 2 months for ethane, 2 weeks for propane"

Yes, they react (oxidize), one carbon at a time; what you have to understand is that butane will break down to propane and CO2, then down to ethane and CO2, and finally, methane and CO2 (The methane will oxidize to CO2 eventually). So, to say that butane or propane is not as much a problem as methane is more than a little misleading. Certainly, mass for mass, methane will stay in the atmosphere longer. But molecule for molecule, propane or butane - because they decompose to methane before completely oxidizing - will stay in the atmosphere a little bit longer.

A google search located several good references. One listing identifies propane as a "green gas". Further investigation might show details I did not review. By the titles, it shows propane is/has been used as a propellant in some spray can products. AGW unknown. In an opinion, these products make good flames, but not useful for escaping gravity. Once off the earth, they would likely freeze (an opinion I cannot substantiate).

Do you mean as an aerosol propellant? The main CFC replacement I know about actually is CO2, but the quantity is small compared to just a few autos (unless you're one of those who thinks exclusively in terms of farts and belches)

Shhhh.... If some consensus tells them that propane is bad for the environment, they will want to ban ourdoor BBQ's!