I doubt Interplay will be using any element of the FO:3 game engine, so licensing of that would be a rather moot point, to my way of thinking, I'm expecting to see a game system similar to FO:1, and FO:2 with obvious graphical improvements, and slight engine overhaul to be made more adaptive to a MMO environment.
Of course the other question is even if Interplay can use additions to the IP from FO:3.... do you think they will? I've not heard any kind of feedback from developers of the earlier FO games in response to FO:3 so I can't personally forsee where they see where the MMO is going to go.
Also does Bethesda retain rights to produce a 4th single player Fallout Title?