Today I sent message to Richard Stallman (rms@gnu.org), asking him whether an idea I have for a GPL-friendly business model is legal or not. It'll be interesting to read his reply. His autoreply was cute too.

You can read the letter if you like.

Update: In short, Stallman didn't like it, and considers it a violation of the GPL. The way he worded it makes it pretty clear that it probably is - what I was thinking of is essentially a contract that requires the user to forfeit his GPL-rights for the duration of the contract, which can be terminated at any time (by the user). If I understand his reply correctly, the freedom to terminate doesn't make up for the restrictions involved. This makes sense, although I think it's a pretty grey area.