Miles Johnson from the Inside AdWords crew explains how it all works:

Mobile ValueTrack works the same way it does for search and content: by automatically adding a tag to your website URL. To use this feature, you just need to add the Mobile ValueTrack parameter: {ifmobile:NewTagName} to the "Destination URL" field when you create a text ad.

The best way for advertisers to take advantage of this feature is to use automatic re-directs to mobile optimized sites or third-party tracking tags.

It is estimated that advertisers will spend over $1.2 billion by 2015 on mobile display ads, so it's good to see that Google is helping marketers calculate mobile display ROIs.

Google have advised that this tracking works for both WAP mobile ads and regular text and image ads on iPhones and other mobile devices with full Internet browsers. Google provides more information about this tracking service here.