Looks like The Beatles: Rock Band will in fact include the rumored multi-vocal additions:

http://kotaku.com/5221506/retail-listings-confirm-the-beatles-rock-band-3+part-harmonies

"The Beatles: Rock Band is the music game experience that allows players to follow the legendary career of The Beatles from Liverpool, to Shea Stadium to Abbey Road. Featuring the ability to play drums, lead guitar, bass guitar or sing three part harmony with up to three microphones, The Beatles: Rock Band will focus on authentically recreating the feeling of performing The Beatles music on stage and in the studio."

Sweeeetness. (And that means Ringo sucks even more)