Interplay is sneaky by making the MMO before it has the rights. They can say to the judge that they have already invested all this time and money into the game and to lose it would cause them to go under. The judge will be more likely to side with Interplay or force the companies to compromise. It has worked before in a few cases and in law, precedence is everything.
It all depends on the judge. He could also be of the opinion; if you sell a car to someone, it is no longer yours. You can't expect to drive it again whenever you feel like.
In my own opinion, Bethesda's position seems a little weak. After reading the patent office's definitions for trademarks and intellectual property, it seems that the trademark encompasses the Fallout IP. Depending on how you interpret it, a trademark is either a symbol that can be used to describe a product or a symbol that describes a specific product (in this case the Fallout IP). The latter interpretation would imply rights to the IP. Intellectual properties are "creative works or ideas embodied in a form that can be shared or can enable others to recreate, emulate, or manufacture them." I would argue that the IP is "embodied" in the form of the Fallout trademark, so rights to the trademark means rights to the IP.
Don't take this rant to mean I want Interplay to get the rights. I think neither should have them. Bethesda botched up the series and the magic that was Black Isle Studios will never happen again (although Bioware comes close sometimes).
However, Bethesda's success would be the worst case scenario. Imagine, if you will, George Lucas gives someone the rights to the Star Wars trademark to make a movie. Now, according to Bethesda, this only includes the name and logo. It can't use any material from the Star Wars universe. So the makers of the movie would either have to scrap it or make a movie about a runaway freight train full of toxic waste or a guy getting zapped by a beam and winding up in a computer world and because it carries the Star Wars brand, it is canon. That is what would happen to Fallout if Bethesda wins this case.