Seriously. When I read that, I thought it was a good move all the way around. Apparently there's a deleted scene where his last name is confirmed. Ultimately, it doesn't actually contradict anything so it still works. Some would say that the deleted scene reveals his last name, but since hit men don't exactly use their real names anyway, it's a moot point.
Personally, I like the idea that "collateral" comes from Jamie Foxx's line, how he's just collateral in all this. I like movies where the title isn't obviously connected to things in the movie, or you have to take a second to figure out why it is what it is.