the NBA has issued an official statement about this:
NEW YORK, May 6, 2008 – Joel Litvin, NBA President, League and Basketball Operations, issued the following statement today regarding the inadvertent clock stoppage that occurred at the end of the third quarter of the Detroit Pistons’ 100-93 win over the Orlando Magic last night at the Palace of Auburn Hills in Game 2 of the Eastern Conference Semifinals:

“After reviewing the video of last night’s Pistons-Magic game, we determined that the play that concluded with Chauncey Billups’ three-point field goal at the end of the third quarter took approximately 5.7 seconds. Because there were only 5.1 seconds remaining in the quarter when the play began, the shot would not have counted had the clock continued to run.

read more: http://www.nba.com/news/clock_statement_080506.html