During my recent Delta Connet flight from Las to Lax, I saw a Chinese guy (Asian men holding a Chinese newspaper) upgraded himself. He just moved without asking or being asked after the plane door closed.
To be honest, there is little difference between regular Y seat and C seat on Delta Connect. But he got a muffin instead of peanuts. :-)
So, self upgrade also happened on U.S. carriers and if one is brave enough...