They're not exactly common in the Boston area, but they're here, and new ones are getting added (mainly in Cambridge, as far as I can tell). At least one even has separate traffic lights that turn green for bikes a few seconds early.

FWIW I tend to stick to the roads anyway, though, because the segregated bike lanes often have pedestrians and runners in them regardless, and the paths often get cracked from tree roots and whatnot. Most drivers don't seem to care if I'm not on the dedicated path, so long as I'm hauling and not riding crazy.

New York added an incomplete network of 'protected bike lanes', as they're called, a few years ago that we are working to expand. Chicago has likewise started building them, and after that they've been filtering down to places like Pittsburgh and Austin.