All 767-300ER 3-Class are operative. Same with the 747s.
Of the 777s, all are operative except for the 28XX series (N216-N229UA) and, for some reason, 2508 (N208UA). This encompasses 16 aircraft out of 60.
Don't know why some 757s and A320s are not operative yet, and I don't understand why none of the A319s have them, given that a lot of them are newer than many A320s, and that they tend to do long stage lengths just like the A320. I can understand no need for them on the guppies, but the A319s??
FYI, on an unrelated -but fleet centric- note, Decor 5.6 is confirmed to be on several 757s... I flew on N592UA today, and she had it. Lovely!
And for those who think "who cares?!" about interiors, it is indeed a very important part of United's marketing strategy, as evaluating it is one of the most prominent questions on United's latest in-flight survey.