Barcode readers for mobile device displays are growing and is an interesting market, I think. In short, it’s hard to read from a tiny LCD screen, but the technology is catching up and a few solutions have appeared on the market.
A similar technology is already deployed by Spanair on their spanair.mobi. The service is active only in limited number of airports, but gives an idea of how useful it can be. If successful it will be extended, of course.