Our family was flying last weekend and at Miami were were queued up for boarding near the front of the line but not at the very front. A man stepped in front of us. I said "Excuse me, we are in line" he said "I am in First Class" in a very nasty tone. I said "So are we" and he kind of mumbled under his breath and stepped to the side, when boarding was called he crowded in right behind us.

So yes- other passengers do it all the time, Gate Agents should not. I never see them police the line at all until they actually scan the boarding passes.