Most mtn. bike rims are smaller than 650B (559mm ERD vs. 584 ERD), so 650B tires wouldn't work. In terms of rims, I bought a pair of Sun CR-18s from Harris, and as many others have noted, they're an annoyingly tight squeeze for mounting tires (i.e., they've been made a bit too large). I don't believe Harris is carrying them any longer until Sun improves on that design. In terms of tires, Panaracer Col de la Vie are ones lots of folks like and can be had pretty inexpensively. Cycles Valhalla is also selling basic 650B tires at a very reasonable price: http://www.cyclesvalhalla.com/650btires.html

Neal