In reply to Miroslav Glavic.

Miroslav - You absolutely could do that, if you wanted to do that, but nothing in the GPL *requires* you to do that. It's good business, but not a requirement of the license.

So, again, I'd spend time on a carefully worded support agreement, if anything. Honestly, I think in actual practice, few people will steal and resell the work/code, in spite of what people are afraid they'll do.