I like the idea of this but I think something in reverse would be much more useful. For example, a keychain application on the phone that can log the user into a website on any other device by scanning the barcode.
It's currently in the App Store review queue but I would be happy to provide a private build for those interested.
Works on any site, no plugins or backend/server modificatons required.
If you're still interested, mail me at info@seqrentry.net and I'll make sure you get a copy as soon as it's in a workable state.