I just put a post on another thread, but we're at 70% completion for the Next mods. This is for all Domestic Mainland Narrowbodys, does not count the 757-200s and Guam fleet.

Regarding DTV, the 737-700 is the only fleet that gives you good odds of having access. There are only 3 737-800s in service with DTV and about 16 737-900s with DTV, most of them being the non-ERs. There might be 1 or 2 more 737-900s with DTV, since the fleet site shows a few planes with Next, but notes that passengers have reported that to be incorrect.

As I mentioned in the other thread, the focus seems to be on the 737-900s so we should be even closer to 100% by the end of this year.