If one of Wikipedia's enemies wants to spend lots of money causing trouble, there are plenty of issues they could raise right now, and, as I noted before, there's a good chance they could win (though the monetary award in any case would probably be negligible).
There's a chance they could win now with some serious effort. If we start openly violating people's copyright on the grounds that they didn't explicitly say we couldn't, I'm not sure it would take much effort at all (Mike says it would, but I'd like a second opinion, or at least some references).
Actually, I will add this. It depends how open and blatant the violation is. If it's just a matter of accepting new contributions under a copyleft free license in the same spirit as the GFDL, and combining these new contributions with the old ones under the GFDL, is it really that much more open and blatant than not following the terms of the GFDL in the first place?