Rule #1 - Gifts don't go to the wedding (other than cards). Bring gifts creates another thing for the couple or their parents to have to take care of that night and according to the etiquette queens the gifts should be sent to the brides home. Most people (around here) just give $$ for the wedding gift in which case see Rule #2
Rule #2 - It's not a good idea to put cash in the mail and it's not a good idea to hand someone a card full of cash or stick it in some box at a wedding either. Cash is the easiest thing to steal, how would you trace that? A check is much easier to trace. There is an identity theft issue sure, but there is also one everytime you pay for anything with a check.
I am now stepping down off of my soap box and preparing to be flamed