OXM UK reports that a "chance meeting" was the catalyst behind The Beatles licensing their tracks for use in Rock Band 2.
The story came from Harmonix PR John Drake, who told OXM that Van Toffler, the president of MTV Networks bumped into George Harrison's family while they were on vacation. It turned out they were big Rock Band fans and that was the driving force behind The Beatles' tracks being licensed for Harmonix to use.