L.A.
You need to write it on the invitations, otherwise, people will spend money on gifts and then when they get to the shower, they will be expected to give money as well.
So word it short and sweet, something like:
"Because they will not have a place to set up their home for a while due to his military leave to Iraq, gift cards are greatly appreciated instead of gifts to help them start out their future together."
Usually, people want direction on what to buy anyways, hence store registries. I know I wouldn't be offended to read that given their situation.