I flew with Lufthansa who are now offering mobile boarding passes. You can get a test boarding pass sent to your phone by SMS or email if you want to see how it works. In my case, I went to the boarding pass URL in a desktop browser, saved the page as a PDF and slapped that onto my iPod Touch using FileMagnet.
It worked a treat. When I was going through security and when I was boarding the plane, I showed my passport and my iPod. They took the iPod, put it under the same scanner they were using for paper boarding passes and scanned the QR code on the screen.
I like paper as much as the next dead-tree fetishist but this was one instance where I was happy to go digital.