With yesterday's news that Google is currently testing an icon indicating that a website is not mobile-friendly, it feels as though it is only a matter of time before we see some sort of mobile-compatibility icon in the Search Engine Results Pages (SERPs).
As detailed at the bottom of yesterday's news, Google has also trialled an icon indicating that a site is mobile-friendly. This sort of A/B testing is a common Google go-to action when they know they want to include something and are just ironing out the details of how to make the user experience tiptop. That the feature has reached this stage suggests that one or both of the icons are not far from full implementation into the SERPs.
For anyone who has not yet gone responsive with their website, this is just the latest in a long line of actions from Google that should really push you towards that route. For a while now is has been accepted that having a responsive site is a ranking factor in Google (it is Google's official recommendation for the development of mobile sites) and the implementation of the icon(s) suggests that non-responsive websites will not only be impacted negatively on rank (where you appear in the SERPs) but also in performance (how many people actually click through to your site), with the icons likely to reduce clicks from mobile devices for some sites.
This is compared to what Google was testing a couple weeks ago with a mobile friendly icon in the mobile Google search results.