He left town last Thursday, in a flurry of "I'll have my cell if you need anything, but I'm sure you can handle it." We can, of course, we carry him most days, except for the things he keeps from us entirely. Count on those to be the things that blow up.
Boss trust is a funny kind of trust. Equal parts "Can't you just handle it?" and "You should have known better."
Boiled down, "cover for me," means "do what I would do, but I wouldn't insult you by leaving a list." They'll find other ways to do that.
They will send you to meetings you never knew they went to. The people in the meetings are bound to be tiers above you, so they don't know who you are, and that's just as well because then they don't know how to take you. One plus of today's casual work environment is that execs and drones look a lot alike. When cornered to speak for your department, affect a scowl and say, "I think we both know how Bob would answer that." Hope they don't call your bluff.
They will mistype your name or phone number on the out of office message so no one can really contact you.
They will override your decisions without even knowing what they were. The miracle that is Treo. It's a dance that goes like this: email to Boss bounces an out of office, sender deciphers how to reach the Backup, Backup provides assistance, Boss emails an hour later responding to the Sender and cc'ing the Backup. The CC of the backup can not be accidental, and the effort says, "here's what I want you to do, my backup. I want you to answer my mail, not lead the group."
They will bring you a souvenier, like you are their nephew or their dogsitter. Since you won't get a vacation, enjoy this souvenier of mine.
They will nod sympathetically when you complain later about this, and respond by naming a different back-up next time.
Maybe that's what you wanted all along.