From what I gathered across the Bare Knuckle blog there is a very sweet 650b ZR Cycles floating around that was built late last summer.

The build looks solid - Shimano XTR drive train, Hope headset and hubs, Rock Shox Reba fork, slick Avid Black Box levers and BB7 disc brakes. Heavy Mettle tubes and prototype Baby Bear 2.3 tires. 25 lbs without any super crazy light parts is pretty good for a “heavier” wheel size of a 650b.