Wow, what a thread. I've read about 30% of it, so hopefully my points haven't been hashed to death already. My post got kinda long, so I've put some of the key points in italics.
At first I thought of the same way around the issue (two companies with a licensing agreement) of requiring a publisher to drop OGL to use 4e, if that is a requirement. (It seems we're still waiting to hear for sure.) but these are smart people and they'll word the license to preserve their intent and they'll use their catch-all if someone does violate the intent.
But more importantly/interestingly, why do you need to follow the GSL to make 4e compatible material? I'm sure someone could come up with some text to add to a product's cover that lets people know that the content inside works with a particular ruleset (or maybe several) without violating WotC's trademarks and copyrights. Someone did a "4 any 3dition" logo a few months back for example. Something like "Includes stats compatible with the latest version of the world's most famous fantasy role-playing game" would let everyone know they can use that adventure, setting, sourcebook, etc. with 4e while hopefully not violating any trademarks/copyright. (I'm not a lawyer so I'm not confident my example is OK, but my point is a lawyer could think up something like it.) We are a pretty sophisticated, intelligent market, I think.
The fact that you might have monsters and characters using stats based on the 4e rules shouldn't be an issue if my understanding is correct. That is: rules/concepts can't be copyrighted, just the specific text of the rules. (After all most RPGs borrow heavily from other RPGs. After all, how many use some combination of the core 6 abilities--maybe with one or two renamed and one dropped or one added. Examples of borrowing an idea and then tweaking it a little are more common than original ideas.) So I don't understand/believe that WotC could come after someone just for saying that the character Suzy has a Strength of 10, Con of 12... Will Defense of 15... etc. (If you copy the exact stats of a monster that probably is an issue, but you could easily just refer the reader to "your game system's monster guide.")
If someone is a lawyer in these matters and knows otherwise, please say so.
The d20/GSL licenses give that content/book some recognition in the market and the ability to copy portions (the extent of copying using the GSL is TBD) of the rules/stats. So based on what I wrote above, the recognition might be lost to a degree, but it could still be largely preserved. Copying rules/stats is an issue, but as I wrote above you could refer the reader in a generic way to the proper book and you should be free to list your own stats for your own custom creatures/monsters. Likewise if you're developing a sourcebook (such as a book of new classes) you should be able to spell all your rules for the classes and simply refer in a generic way to the creation and leveling-up portions, etc.
Now that I've hashed this out, I could see some publishers continuing to publish material based off the OGL, but include an appendix or shaded box or online reference so the same material can be played using 3.75-ish rules (where a publisher takes the OGL, maybe borrows a few ideas from 4e and adds a few of their own ideas), and also includes info (stats, new spell, etc.) for using the material with 4e while only referring to 4e in a generic way. So publisher x could put out a book that is a fantasy pantheon for example. Within the chapters of the book every god has core OGL-based stats. But they know publisher y's ruleset is good and publisher x has a license with publisher y (an open license or not) so they include an appendix or on-line resource or shaded boxes for each god's stats (or just the needed differences) using publisher y's rules. And the same pantheon book could also have the stats for the gods in the 4e rules as long as 4e is referred to generically.)
Of course, run this by an expert first.
