All the ATT/Apple lines are really civilized, especially at night. Everyone is usually sitting around fooling with their tech or chatting with their friends. Getting out of line to get food isn't ever really a problem, as long as you make a point to get back in line in a reasonable amount of time. If you're going to be gone for more than ~30 minutes, offer to bring back a box-o-joe and you'll make a dozen or so new friends.
Now, as it gets within two hours of launch time, everything changes. The last-minute, rowdy teenager types show up and start getting all noisy, but you should be pretty isolated from these jackasses.
You might even consider bringing numbered index cards to hand out, if you're in the front of the line. This will eliminate last-minute save-my-spot cutting and doubling/tripling up of friends, though the addition of a spouse or child should be deemed perfectly acceptable. The ATT/Apple people are usually impressed with the pre-opening organization of numbered lines, and will generally enforce the index-card ticket system if those in the front of the line agree it was done fairly.
Everyone is there for the same thing. They want an iPhone and they don't want to be screwed out of one, but no one wants to be miserable.