Are you familiar with Bill Maher's New Rules segment? Whether you are or not, here's my new wedding rule:
Cash registries are tacky. Everyone wants cash or checks as wedding gifts - who wouldn't want $100 to use on whatever you want? But you cannot register for money. You can't put a button on your wedding website directing guests to PayPal. You can't tell people to just write you a check when they ask what you'd like as a wedding gift.
Register for gifts either at a brick and mortar store or online or even for your honeymoon - though I'm not huge on these either. And choose a store or site with a good return or exchange policy so if you do get something you don't want, need or like you can exchange it or return it for a store credit.
The only excuse for this situation is if you and your groom are moving out of the country and it'll be impossible to move everything to your new home. I know of two couples who had this issue and one was in the military.
So register at a store - or two or three - and if you really want cash gifts keep your fingers crossed that your guests are the cash/check-writing crowd.