Google has been doing this for about a year now using a tiny icon on mobile screens notifying the end user that the website is mobile friendly, however they changed this with actual real estate now that displays “Mobile Friendly”
This is a huge change, right now mobile sites are not getting a boost in their actual positions (to any large extent) however the added notification appears to be increasing click though rates. In other words if you are position 4 , you’ll most likely still be 4, but with a click though rate of 5% rather than 3%. It means more traffic.
Below is a screenshot from an iPhone search result. Notice the “Mobile Friendly” tag next to the date?
Clearly having a mobile site will give you an advantage moving forward. With a Rough estimate of nearly 50% of searches taking place on mobile devices in many metro regions it makes for a better user experience with their device of choice.