Now, first off everyone stay calm. It seems that in the past when we dicuss UA and the 737NG everyone goes bananas with some individuals saying why not, and others jumping down their throats with NEVER. First off, UA is a very big fan of the A32X series. They have a large fleet of A320s and a growing fleet of A319s. So, common sense would dictate that they would not be interested in the 737NG. If that was the end of the story, I would whole-heartedly agree. As we all well know, it is not.
United also has a gigantic fleet of 737-300s and 737-500s. The 727-200s are being replaced with more A319s and A320s, but that does not mean that the 737NG doesn't have a future home at UA. The 737NG is highly compatable with the 737 "Classics" and its cockpit can be configured to be just like theres. In fact, this is exactly what WN and CO did with their 737NGs cockpit. Therefore, very little training was neccesary to move 737 "Classic" pilots up to the "NGs".
Now, there are going to be 50 727s that will be retired in 2-3 years. DEN's hot and high conditions must be looked at. This is where I think the door is open for Boeing. With the winglets that are currently available on the 737NG line, those beautiful birds can really get up. I am not saying the A32X can't, but the Boeing's really jump into the air.
My main point is that while United is a large operator of the A32X family, they are a larger operate of the 737-300/500 family. The 737NGs are highly compatible with them and I don't see why in the future UA won't order some.