Meh. Interplay needed money in a bad way. Bethesda came up to them told them they wanted to buy the rights to Fallout. Interplay said they would, but they were working on the MMO for it at the moment. Bethesda said "Oh, that's cool. How about we just buy the rights, but you get to keep MMO rights to it as long as you've made good progress on it by a couple of years from now. If you don't, then we get the full rights." Interplay said yes.
It's been a couple years and a few months change now, and it doesn't seem any further along then it was then.
Sad to say, but this isn't any type of evil ninja lawyer legal loophole sorcery on Bethesda's part to evilly gain control of the Fallout MMO rights. They put down the ground rules in black and white when they made the agreement. Hell, Beth even gave them about an extra half a year. Interplay didn't even need to have it -done-, just have had decent progress on it. They haven't, so an agreement's an agreement. If hardcore Fallout fans want to blame someone, blame Interplay for agreeing to the deal in the first place.