A colleague asked me the other day where she should get a bridal shower gift for a bride when there isn't registry information included in the shower invite. I told her that when she RSVPs she should ask the host where the bride is registered. If for some reason the host isn't sure than she can either ask the groom - since he'll know and you won't ruin the surprise by talking to him - or check out the couple's wedding website.
Technically it's not proper to include gifting information in an invitation - that's why you don't include it in a wedding invite - but I think that a shower is different. Simply speaking it's expected that guests will bring gifts. The whole point is to "shower the bride" with items to use in her married home.
So don't be afraid to include information on the bridal shower invite or the insert cards provided by the stores the couple registers at. You'll save your guests and yourself a lot of trouble by making it easy for them to shop.
And as for my colleague, I told her if the couple wasn't registered anywhere than she would be safe with a product or gift certificate to a chain store that takes returns and exchanges easily in case the bride doesn't want or need her gift. And if it's a gift card that she buys, make sure it's from a store with a large selection of products, such as Bloomingdale's, Target or Williams-Sonoma.