The recaptcha box isn't displaying on WP8 with official IE mobile browser.
Just go with the device (I tried with wp8) to the official demo page:
http://www.google.com/recaptcha/api2/demo
and you will see that the recaptcha box isn't displaying.
Problem solved: I'm using a Windows Phone without a SIM card to develop with, but when I first configured the phone 1 year ago, I set to automatically synchronize date and time. (like everyone does)
However, without a SIM card and over WiFi, the phone doesn't sync the date and time, and therefore the https certificate from google appeared as expired and the iframe didn't load.
Adjusting the date and the time manually solved the problem.