Since the bike industry decided that introducing new wheel sizes was a good way to generate sales! No frame swaps any more....you now need forks, wheels and tyres.
Those that missed the 29er band wagon are pushing the 650b one. Look at Santa Cruz, the only 26er will be the V10 at the rate their going. Apparently, they've been inundated with demands for the 'tweener wheels despite lads of people having never ridden them! The Americans in particular seem really wiling to adopt them and think that they will be the best thing ever.