db4o has just published a paper written by a law firm Greenberg Traurig on GPL and dual licensing. This is core of their community and commercial business model, just as it is for Esper and EsperTech.
That is one of the best and most of to date resources on GPL and dual licensing I have ever read, and I 'd encourage anyone working and using GPL code to read and understand it. It also does cover GPL and hosted projects (Application Service Provider), work for hire and distributor (of course) situation.
You may read there: You may hear people argue about whether the GPL is a contract or not. The important thing to understand is that whether it is or not, if you accept code under GPL terms you must abide by the GPL. If you breach the GPL, your rights under GPL license terminate, and you have no right to use the code at all. If this happens, you will be infringing the licensor’s copyright, and copyright infringement puts you at risk of civil damages or even criminal culpability with the possibility of heavy fines.
Read it from there or PDF here directly from db4o website.
If you want to read further on GPL and dual licensing, the following law blog is quite interesting as well.