I agree that Moveon.org was baseless in their accusation and anti-Bush types are using this as a scare tactic. That "8 million could lose pay" doesn't seem to be supported by any fact.
But, interestingly enough, just before it points out the factual discrepancy, Factcheck.org says this: "Which is closer to the truth, EPI's 8-million figure or the Labor Department's 644,000? In fact there are no solid figures on how many workers qualify for overtime now, so all estimates involve more than the usual amount of educated guesswork."
It seems that the fact remains that there is going to be a net loss in overtime protection.