Quick Q. re. SFO
 

Can I walk between the AA and UA gates without going outside security?

No. Pretty sure, but not 100%

Yes

Another yes.

There is an airside walkway between the AA pier (gates numbered in the 60s) and the UA pier (gates numbered in the 70s and 80s). There is a good selection of shops and eateries.

There are four airside zones at SFO:

1) International Concourse G, the United and American gates (hallway connects G with Term. 3),
2) International Concourse A (Virgin America, JetBlue and various international carriers),
3) Boarding Area B in Terminal 1 (Continental, Alaska and a few other domestics), and
4) Boarding Area C in Terminal 1 (mostly Delta).

I'll pile on with a definitive Yes

Great, thanks for all the responses!

I often take AA flights out of SFO and eat over at the UA side. The food choices are much better over there.

You mean those are called "choices" on the AA side? :p