a lot of great comments have been already said, but i'd like to add that the "cross" isn't just a christian thing.
The Greeks depicted Bacchus with crosses. Mexico had crosses centuries before the Spaniards arrived. The Egyptians and the Hindus used crosses as well.
the Christian use of the cross didn't even begin until the time of Constantine, three centuries after Christ.
in fact here is a great webpage on a bunch of different crosseshttp://www.designboom.com/history/cross_symbol.html
just something i felt the need to point out.
edited to fix url address