What the...?
Now we are talking serious, baby!
http://www.flightsimulatornetwork.com/topic/2817-pmdg-for-x-plane-10/
http://board.vacc-sag.org/42/49152/
Not completely the same info, the first saying they have a working beta for the 737ngx, the second - and newer - saying they are still evaluating which plane to do for X-plane, with the 737ngx being one of the two options mentioned by name.
Anyhow, it will still take a long time, I think. The 737ngx would be an extremely complex package to do.
However, PMDG seems to worry about X-Plane 10 becoming a healthy and thriving platform for which to develope would be an enterprise that returns the needed commercial success. There is always the chance that they cancel their engagement, if they assess XP10 to be not promising enough, commercially. I think it depends on other heavy weight developers also boarding the X-plane. Seen this way, the unimpressiveness of recent release "Flight" by Microsoft may work in support of X-Plane, since Microsoft failed to introduce an alternative in functionality that is technically up top date with XP10. While Flight seems to look nice, its content seems to be anything but impressive, reviews say.