This is very similar to what Armory is currently doing for offline wallets but the transference method through usb stick is not as simple as the QR code,
But Armory's offline signing isn't standalone.
That's all the offline computer does: lets you confirm the transaction details, and sign it.
It still needs the connected instance to create the unsigned transaction which is than brought over to the offline system for signing.
And here's the reason you wouldn't want to trust the merchant to build the unsigned transaction for you:
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http://bitcointalk.org/index.php?topic=68482.msg1187718#msg1187718I haven't looked at this closely but the approach is interesting
VisualBTC - Android-based hardware offline wallet using animated QR codes -
http://bitcointalk.org/index.php?topic=210371.0