This is the bit where yet another uninformed non-lawyer spouts off. Don’t say I didn’t warn you.
Wizards has released a license for third parties who want to publish 4e compatible work. It’s much more restrictive than the analogous license for 3e, which is Wizard’s right. One of the interesting sections is section 6, reproduced in full down below, but which in short says that once you publish a work using the new license, you cannot publish an OGL version of that work or of any works in the same product line.
I.e., Green Ronin has this popular and successful Freeport product line. They’ve published a generic book that describes Freeport, and they plan to publish system-specific books for a number of systems which contain stats and mechanics. So far they’ve done three of those; one for Savage Worlds, one for True20, and one for D20. The latter two are OGL products.
If Green Ronin wanted to do a 4e system book for Freeport, they could. But they’d have to stop publishing the True20 and D20 system books. And they’re not allowed, by the terms of the license, to go back and republish those even if they stop publishing the 4e book.
Now the uninformed question: does this constitute restraint of trade? (Oh, god, linking to Wikipedia in relation to legal questions. I’m going to hell. WINAL, as they say.) I note that Wikipedia claims that restraint of trade is generally applied to post-employment clauses in employment contracts and conditions on sale of business. The former is why it’s hard to enforce non-competes in California employment contracts, for example.
So probably not; but it’s an interesting question. My instinct is that Wizards would argue that the clause acts to protect their interest in their logo and brand — another clause sets standards of quality for licensed material, and allowing a third party to publish material in the same product line which does not fall under those quality standards perhaps risks brand dilution.
But yeah, not a lawyer.