Same here, updating as typing. Make sure your 3gs is off and your Wifi is on on your HD7. The first couple times I had both off and nothing. Once I read through the thread I learned I needed to have Wifi on on the HD7 and the first try it worked.
Can you post your radio revision number and such? I'm thinking about trying this method but worried that it isn't the device specific build which is what I want to make sure it contains the fixes HTC has made and the radio firmware update that T-Mobile added as well...