I often volunteer to be bumped. At United, it definitely goes by your status. There could be a dozen people on the list when I get to the gate, but once I am put on the list they always say that I am on the top of the list since I am a 1K. I have looked at the VDB list also on the computer and see that it goes by status, and if one has equal status then it goes by the fare basis. At American, where I am a Platinum, it goes on a first-come, first-served basis. There is no benefit to being a Platinum or Gold on AA with respect to bumps (unfortunately).