Cash is always appreciated (and more versatile than a gift card, too). Some people register for high-end gifts for groups, like their co-workers, to pitch in on.
I just got married in December so I have some recent experience with weddings, gifts and registries.
If you get a VISA gift card, it can be used anywhere that takes VISA credit cards so it's VERY flexible and can be used for a variety of things - you can go out to dinner and a movie and just use that one card for both. Or buy table linens and flowers. Whatever. One of the grocery stores where we sometimes shop has a whole kiosk of gift cards including the VISA cards so it's pretty easy to get hold of.
I agree about registering for things like iPhones and stuff.
i just typed a huge amount of ideas and it all got erased....UGH
I try to get stuff for weddings that are more meaningful and personable and not-so expensive. I use creativity rather than money, cuz I have lots of former and not much of the later. LOL