3/26/2012: 641 LAX-HNL ^ , and was announced by the captain that it would be there a half hour out of LAX and an hour into HNL (I listened for about five minutes on the way out and then didn't turn it on again until descent). He was advertising it; pretty cool.
3/27/2012: 969 HNL-LAX ^ , no announcement just there into LAX (didn't try to listen on the way out)
2/29/2012: 927 SFO-LAX ^ , was the last person to board, no idea if it was announced or not, just lucky to have made the last flight of the day given the blizzard delay from SLC...
I think that is all of the mainline UA United I have flown this year... Wednesday night SFO-LAX was a CO flight, and there was no channel nine in existence. Obviously CO didn't, and I think doesn't, have the ATC channel, but was there a 9 with something before?