Given that PAX East is in Boston, Harmonix naturally reps hard there. They or people related to them will be hosting or appearing at several panels during PAX East. Harmonix has confirmed that they will be announcing a new title at one of the panels. Source (Note: I linked the story from a Rock Band Fansite but be aware that they have not said whether the new title will be a Rock Band, Dance Central or completely brand new game.)

If it's at a panel, I'd have to guess that it's something on the scale of VidRhythm. If it were a big budget console game, they'd probably announce it in a flashier way.