How soon before library barcodes are on your cell phone?

Ars Technical reports that American Airlines will start rolling out cell phone boarding passes today at O’Hare international airport.  Instead of printing your boarding pass* you’ll get a barcode sent to your cell which you can bring to the airport. TSA can scan it and allow you to your gate–you’ll still need an ID, of course.  How soon before we can get a barcode on our cell phones for everything–supermarket key tags that get you the lower prices, and everything else including…library cards? *See “Why is Printing so Hard?” by Ed Felton, Princeton Professor and apparent leading candidate for Obama’s new Chief Technology Officer position.