It really is an issue of WotC hurting themselves with a restrictive GSL and what came before. Had the 3.x OGL been this restrictive less companies might have jumped on, but if they wanted to publish 3.x compatible material, they would have had to, because back then people were used to "one company, it's own rule system" people didn't look for AD&D 2nd edition comptatible books from anyone but TSR (and in the later years WotC) but the OGL changed that and everyone could see the logo. Now the logo isn't important, because people will just be looking to see that it is 4e compatible by opening it up, or looking on the back and reading the description that might mention "compatible with the newest edition of the most popular fantasy role playing game" or something along those lines or just an uncopywrited 4e.
People will be looking for that, and won't care if it bears WotC's official logo. Though if people that are circimventing the GSL produce unplaysted and unbalanced books (why I stopped buying the Mongoose Quintessential Books) they might come to again.
Still, that said, I LOVE 4th edition, I can't wait to get 4e stuff and with so little out from WotC you can bet I'll be snatching up the other publishers, but I don't care if it's GSL or not.
I'm glad to find even these publishers listed, all I knew about was Goodman Games. I also apologize if I got a bit off topic. Woot to Necromancer games too... though I wish they would publish their Tome (need more expansive monster books), with or without GSL.