Today I passed through a Border Patrol checkpoint, one of several that I have been through. They seem to be permanent checkpoints, but they aren't actually at the border crossings. I think they are just to systematically check for any non-citizens.
This particular check was interesting because it was in the middle of nowhere so they were pretty surprised to see a biker. It was at least 150 miles of nothingness between the towns, so they wanted to make sure I had enough water, etc. Since I knew the distance, I had prepared by taking an extra gallon of water with me. I think they didn't realize the kind of distance that someone can cover on a bike, thinking that I would be out for a week or so between towns. They did offer me extra water just in case, but I didn't need any more, so after they confirmed I was a US citizen they let me pass.