While it's possible that it could be overturned, and I'm no lawyer, I've read the thing enough to know that it's not probable, and there's been so much language for perpetuity put into it that I would put complete trust in it (if I ever had anything I wanted to publish).
Ryan and his lawyers really did their homework when they put that thing together. Near as I can figure it, Someone would have to prove that Ryan and crew never had the authority to establish that license in the first place, in order to overturn it, and that would mean that Hasbro/WotC or whoever don't have legal ownership over the property even now...
The big question isn't so much whether it could be overturned as its relevance to the gaming market at large when a 4th edition of D&D is released. If a 4E was so demonstrably better that everyone just HAD to switch to it, then the point is moot; but if WotC didn't do their PR job successfully enough, then the switch would only be half-hearted by the fanbase, leading to a gradual decline a la 2nd edition days.