Any mountain bike that uses 650B wheels could be called a 650B mountain bike.
So why 650B? The people behind the 650B movement claim that with 650B tires you get all of the same advantages of the 29" movement (lower rolling resistance, better traction, smoother ride, etc.) with less of the disadvantages (geometry limitations, toe clearance issues, higher center of gravity, suspension travel limitations).
So why aren't we seeing a lot of 650B mountain bikes out there? Well, the biggest thing holding it all up right now (spring 2008) is that no major fork manufacturers are building compatible suspension forks. It's a bit of an investment on their part and it seems they have some more important things to do. Some standard 26" forks will work, but they are certainly not designed to for them, and the manufacturers won't approve them for 650B wheels (with so little tire clearance, this probably a good idea).
There are a very small amount of 650B mountain bikes available now, and there will be some 650B bikes out in 2009, but I think 2010 will see some real production, with hopefully some real fork options.