It hasn't been done well. The one company that has been dong this is myregistry.com. If you've ever used that service you know it's just begging for a better-designed competitor.
I was really going to implement this when I got married a few months ago but couldn't find an elegant way to confirm when a guest had purchased a gift. Sending an e-mail is OK but it's not 100% accurate. This can lead to duplicate gifts which are a big pain.
MyRegistry has done this, the big thing they haven't done it price comparison, and their design is absolutely begging for someone to do it better. We were on HN a few days ago, and hoping to do just that. We let users price compare while also integrating with traditional registries (registrystop). http://news.ycombinator.com/item?id=2136619
I was really going to implement this when I got married a few months ago but couldn't find an elegant way to confirm when a guest had purchased a gift. Sending an e-mail is OK but it's not 100% accurate. This can lead to duplicate gifts which are a big pain.