Digikey has them too in low and high quantities. From what I heard from my Cree rep. they said that it was to actually replace the XP-E HEW not necessarily the XP-G (although it could i guess). They said something like XP-G performance for less than XP-E pricing.
Regardless, should be an interesting LED to watch as performance and technology increases.
I just ordered a few from Cutter Electronics in Australia. Shipping to the US will make you **** bricks, but they do have R4 bins in stock mounted on 20mm stars.
Also, the XB-D appears to be more a replacement for the XP-E, with the XT-E being an XP-G replacement.