Bethesda got the license to make Fallout 3 in 2004. They didn't begin to earnestly work on it till after their release of Oblivion in 2006 though. They bought the IP to Fallout in 2007. It's all on the Fallout 3 page, look under development history.
Oblivion was most likely been in the works for close to a year by the time the deal was made as it came out in 2006.
So yeah, Interplay is playing off of the fame that Fallout is running on now under Bethesda. They want it back so they can make money off of it.