Of course cash is acceptable. If a couple is registered, much of the guess work is taken care of. If they're already living on their own, they likely don't need the a "start-up kit" including plates, bedding, and silverware.
Have you ever heard someone say, oh, geeze I wish they'd gotten me that cappucino maker instead of giving cash??
They can get whatever they want with your money.
(btw: I was just in a wedding, and the registry was used mostly for wedding shower gifts etc). After the registery was depleted, some people bought the couple nice, but extravagant gifts that weren't necessary or useful (i.e. a huge $300 punch bowl from Tiffany's). The bowl couldn't be returned, and there was really no place for it. The bride was telling me how she wished people had just given $ at that point. It's always useful, no?