I've tried it a few ways.
I released at the fast blink.
I released at the solid on.
I released after a couple cycles of fast blinking and solid on. (It'll continue through cycles of slow-fast-solid if you hold down the button forever.)
No es bueno.
I think it may be due to my phone.
I deleted the X10 app and reinstalled. It didn't act like it did when I first installed the app. It did not prompt me to scan the QR code. It immediately went to sending data to a non-existent WM100.
I factory reset the app, cleared the app's cache and stored data, then deleted the app and reinstalled. It still acted like it had been configured before.
I think some data is being left behind on the phone.