This question comes up quite often in my PM box and emails, so I figured I would throw out my answers here.
Throttle body spacers (TBS) came about back in the time when TBI engines were the craze. The TBS did indeed help the TBIs as they helped to improve atomization of the fuel by changing the flow of air as the fuel was sprayed before the throttle body. The gains are minimal but are still there.
The TBS are a waste on the newer engines as the fuel is sprayed after the throttle body. The air is already "spun up" as it goes through the throttle body, so the TBS basically duplicates the effort. The TBS won't hurt your engine however, that being said, some folks wind up having problems with the spacer as it is one more thing to wiggle lose and cause leaks.