Google AdWords has announced some recent and upcoming changes to the way search ads display and behave on mobile phones with full browsers (i.e., iPhone and Android).

Most exciting is the new “Click-to-Download” feature, which drives a searcher from your ad to either the iTunes App Store or the Android Marketplace to download your app. Google pulls your app icon from the store into your ad and your display URL is replaced with either “Download from iTunes” or “Download Android App”.

To make a Click-to-Download ad, simply create a normal text ad targeted to high-end mobile devices, but in the destination URL field enter the full link of where the app can be downloaded (i.e.,  http://itunes.apple.com/us/app/example-app-name/id999999999?mt=9).

Note that AdWords policies prevent more than one domain per ad group, so you’ll need different ad groups (or campaigns) for iTunes versus Android. Get more details on creating a Click-to-Download ad from the AdWords Help Center.

There is also a related beta called “App Extensions,” which is a new ad extension a la Call Extensions or Sitelink Extensions. The benefit here is that your headline still links to your website, but your ad is supplemented with a link to download the app.

Google is also changing the background color for mobile search ads, from pale purple to pale yellow.