Thanks again!  So in a shopping cart scenario I can credit a customer.  It sounds like if I want to launch an advertising campaign with a QR code I would need to have have the QR code resolve to a web site and not a bitcoin address directly.

DeathAndTaxes,

 Thank you very much for the explanation.  It sounds like best practices for shopping carts that support btc should include the dynamic generation of unique customer payment adresses.  I suppose adding an arbitrary data field to each transaction, where a customer could reference an order number or assert identification is not currently part of the protocol?

  I have read the faq and googled a bit but was not able to figure this out.  What I am trying to determine is how do I prove to someone via email or their e-commerce website that it was me that sent btc from wallet xyz?

  Is this only possible if the recipient generated a one-time address for me to send to that was presented to only me?

  Or from the merchant perspective, if I advertise wallet number 1234 and a customer sends me an email with an order for a widget and a claim that they sent me payment from wallet 5678, how do I know that the person sending me email sent the payment from 5678 and not someone monitoring the distributed ledger for payments to me?