I'm using a php script to determine if the visitor is browsing from a mobile device or not, and if so, it redirects them to the mobile version of the site, otherwise, sends them to the normal home page.

So the default document at the top of the list in IIS is detect.php

How does Google handle this? When I use Web Master Tools to test, I'm getting no data returned.

Regards

Depends on how the redirect is done.