While not quite confirmation, it looks pretty certain that vocal harmonies, functionality introduced in The Beatles: Rock Band and soon to be seen in Green Day: Rock Band, will also be found in upcoming Rock Band 3.
Speaking at Harmonix's PAX East 2010 panel, Rock Band designers answered questions from the audience, with the stipulation that there were to be no questions about Rock Band 3. That didn't last long, however, with an audience member enquiring whether Green Day: Rock Band songs would still have harmonies when imported into Rock Band 2.
Harmonix Project Lead Chris Foster responded by listing the games which support harmonies, then stated: "We've said Rock Band 3 would have harmonies already, right?"
Foster didn't add anything else about the harmonies functionality.