Multitouch, Microsoft Surface and NUI Ideas.
I like this one a lot for its basic possibilities. The same problems of device integration arise though, the most important being how do you setup the sync with that device in the first place without creating a very user UNfriendly experience? Making this kind of thing work in a public space could be tough. One idea: Use MTags to send the device to a website that sets up the sync.
Yeah, that is an issue. It seems like a better idea for enterprise applications ... or situations where you are pushing information from the handheld device to the larger display.