The flipside also might be interesting. I haven't nitpicked through both licenses to see if there is anything to it, but here's the hypothetical:
I have CompanyKen and released some 3.5 OGC monsters.
CompanyBob comes by, uses my monsters in their Castle DeathSkull adventure under the
OGL.
CompanyBob then decides the
GSL and
4e are the way to go, and use the conversion clause to convert their Castle DeathSkull adventure to
4e and licensed by the
GSL. However, they are now using my monsters, originally released under the
OGL in a product that is no longer licensed by the
OGL.
Can I cry foul?
Again, I haven't picked through the licenses to see if this is addressed, but when I first say the "you're allowed to convert your products to
GSL" clause in there, that was the first question that popped into my head - "Are they permitting CompanyBob to publish CompanyKen's OGC under a new license?"