If the other people who work on the project are in on it, there is no problem. If you got paid for a product, everyone who contributed should get paid. The model is not one guy stiffing contributors. If you are upfront with the dual license, there shouldn't be a problem.

There is a great quote from a band called NOFX that goes "Give him something free, and he'll resell it to the poor". Companies, in general, are not nice. In general, they wont help you out after they sell the code you gave away free – and no one uses "donate" buttons. GPL stops people from making a mint off you free – bottom line if you want to profit off other's sweat, I think you should have to work for it.

…and sending an email isn't that big of a chore.

Do you have the RIGHTS to change the license on a project that consists of other people's work? And furthermore, is it really ethical (or even legal) to charge for a commercial license (ASL, MIT,…) if the project contains contributions from others? Probably not.

I personally don't know the answers to any of the above questions (except the ethics stance), but I think the above considerations make it very clear that you should just select ASL or another KNOWN commercially friendly license.

There was a trend a while ago where open source projects offered code as GPL, and if you wanted to use the code to make money you could pay to get the code under an MIT or commercial freindly license. I think it's a good way to help out the community, while not getting shafted for all your hard work. Your suggestion is even less of a hassle.

I think that is a good idea personally.

GPL is a viral license that infects any code that tries to use GPL. It's a political ideal that has no bearing on the real (commercial) world. I don't like 'viruses'…