Google announced that it will add in the coming weeks a "mobile-friendly" label next to the mobile search results that are optimized to be viewed on a mobile phone. Google will only add this label if the pages don't use plugins like Flash, if text is readable without zooming, content is properly adjusted so that users don't have to scroll horizontally or zoom and links are placed far enough apart so you can tap them.
There's a page that lets you test if a site is mobile-friendly. Google also has some articles that help you create a mobile site.
I think that adding a small icon is a better idea than using the label "mobile-friendly" next to search results. It uses less space and it's easier to find mobile-optimized results.