I used to work for a boss who managed four offices making his time ultra-limited ... he eventually called me one day, and begged me to review their work "unofficially." But there had been a long history in my office of people who would "act like the boss," sans any official title, only to be "beheaded" a few months later for overstepping their bounds.

I told my supervisor that if he wanted me to be "acting boss" in his absence, he needed to let people know officially, and that giving me a new title wouldn't hurt either.