, I did the test. I made sure my wallet was set up on my Nexus 7, including selecting my AMEX card. Then I turned off Wifi and walked across the street and down the block to McDonalds. While in line I opened Wallet on my Nexus 7 and entered my PIN to unlock it. Then when I got to the register, I held the 7 up to the payment device, it showed the NFC wave icon and "Sent", and I got my Oreo McFlurry.
I did get a dialog that says "Network access is required to receive transaction details. If transaction was successful, details will appear as soon as a connection is available." After I got back to the office I turned Wifi back on, went into transactions and hit "Refresh" and there it was: $2.81 at McDonalds.
The cashier was very impressed and the assistant manager asked me to write down the name of the app, so he can see if it will work on his HTC Evo.
So, yes, it works exactly the way I thought it would :-)