"Hideout" Play--Legal or Not?

[reff4e]

This happened in an NC playoff game last night (11-23-07):

Time is running out in first half. Team A on first down runs a dive, one of their players leaves the field, then huddles with TEN players. Ball is whistled ready with about 15 seconds remaining and clock running. A88 comes in from the bench and enters the huddle as A89 leaves the huddle and stops at the sideline. Ball is snapped just before time expires and A89 goes downfield (undetected by defense) and catches a TD pass.

Legal or not? Rules references?