"It was soundly debunked on Mythbusters last season."

I wouldn't rely on Mythbusters for science. Generally they try to reproduce one version of a myth, fail, and announce it "busted". In this case they were unable to control their focus well enough. More people, more training, and more time and experimentation could have helped. Mythbusters tend to be very restricted on time - they probably spent 3 days on this when the ancients would have spent years; and Mythbusters had about 20 staff where the ancients had armies of men to practice with. Scale matters.

There is more on this at the slashdot site. Interestingly, one of the problems is for each soldier to figure out which spot of light corresponds to his mirror/shield. Some kind of aiming mechanism, such as is used by a modern one-handed signalling mirror, would have helped. If they had such, it's been lost.