The mobile boarding pass is “refreshing” because refreshes bring updates of seat-assigment changes, gate updates and upgrades.
Travelers using a mobile phone or web-enabled mobile device can check in for their flights at mobile.united.com starting 24 hours before departure.
Read the full story at http://www.tnooz.com/2010/03/17/mobile/uniteds-mobile-boarding-pass-is-refreshing/