Did this for the Browns-Cowboys opener a few years back. You're not actually allowed on the field, but are in marked out areas on the sidelines and part of the end line. The way the Browns did it, depending on the side you entered from you were only allowed to be on that sideline from about the 20 yard line to the opposite end zone, and about a 1/3 of the way around the end zone. The opposite side had the same restrictions on there side.
Best places to be were right behind the bench, got some great shots of Romo who left his helmet on the bench and came back to grab it, and near the corner of the endzone as the players usually stretch near between the goaline and the 20.
Pretty cool but not as great as it could be. Lots of security being a pain in the ass.