Is there an Android app that will scan a QR code and show the balance? What I'm looking for is an app that lets a person scan a QR code containing a Bitcoin address and then see the balance and transaction history for that address. 

For a specific use case, a mobile phone (or Android tablet, more likely) is going to be made available at an event to allow people to check the balance of their bitcoin address.  Attendees with their own smartphone can use their own mobile wallet app but for those without smartphones and those who don't know (nor care) about bitcoin, the QR code on their badge (or wristband) will work as their wallet for receiving small payments (rewards for their participation).  The badge might be something like  this, for example:


http://www.paymyaddress.com/


The app can be pretty simple -- a text field to enter the firstbits (and a button to optionally allow a scan of a QR code), and then the balance is displayed (with transaction history below, optionally).

Since it would be used by people just walking up, it shouldn't require any registration or login.  Simply scan and view.

BTCBalance.net would work for this but it isn't suited well for use from a mobile.

Has anything that does this been built already?
To get the balance and list of transactions you could do an HTTP get to blockchain.info. E.g:

http://blockchain.info/address/17w9uVZuPgsY7iLBVGB7qTZ1ZiZnTJKHLq