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If it's a regular bar-code, then it won't work (Passbook only generates 2D barcodes). They made a conscious decision, according to the WWDC videos, because the laser-based readers for 1D bar-codes work very poorly on backlit glass screens, but the camera-based readers for 2D bar-codes work magnificently.

So unless Tesco's switching their club code barcode readers, I wouldn't count on it.
That seems shortsighted for mass adoption. 2D imagers can read old school 1D barcodes.

And some laser scanners can read bar codes off some displays.
 
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