Although it may be tempting to let unproven employees "try out" a promotion to see if they'll work out, be careful of the hidden legal risks. If you treat the acting supervisor differently than other promoted employees (i.e., you promoted those people directly, rather than having them jump through the "acting" hoop), you could end up on the wrong end of a discrimination suit.
Although you may think of the trial promotion as a benefit and a vote of confidence, the recipient may claim he is being treated less favorably than others and being asked to do more for the same amount of money.
Recent case: Alltel employee Desi Ledbetter received positive reviews and yearly merit increases. When the company eliminated several jobs, he was named an "acting" manager, assuming the duties of another manager. He was still expected to do the work he had done before, in addition to his new duties.
The promotion wasn't finalized for nearly two years. Ledbetter, a minority, sued when he didn't receive back pay, alleging race discrimination. He told the court that white employees had been promoted within days or weeks of being named to an "acting" position. The court sided with Ledbetter, agreeing that a promotion could be seen as an adverse job action if that promotion meant more responsibility without more pay. (Ledbetter v. Alltel Corporation, No 04-3807/3990, 8th Cir., 2006)
Final tip: Make sure your organization handles promotions with as much care as any other employment decisions. Avoid problems by using a standard time schedule for promotions and salary increases.