[QUOTE=GreerApproach;14471512]
I ask to speak to a supervisor; he disappears and comes back, saying I need to go to the DL counter and get a ticket number.

I oblige, return, and he proceeds to issue me boarding passes. However, he tells me they may have to deny me boarding because they're still oversold and "I'm not yet a UA passenger, I'm still DL's problem.". I say I have a ticket and a confirmed seat assignment... He continues his stance, even going as far as saying that if they deny me boarding, it's delta's issue concerning compensation.

QUOTE]

Stand right there in front of him and call UA corporate on your cell phone for clarification. When he realizes what you are doing, his stance will probably change pretty quickly.