The line “morituri te salutant” is often quoted as the gladiator’s salute. I have seen modern reenactors opening their shows with it. In an article from the Transactions of the American Philological Association, H.J. Leon proved not only that the remark was not common at all, but also that it was used by naumachiarii, men condemned to die in a staged naval battle; besides, the two sources quoting the line refer to the same incident, which suggests that it was only used on that occasion. The article is now online at LacusCurtius; go here.