Google will start indexing pages based on mobile data rather than calculating mobile page rankings based on desktop signals. This means Google rankings will be based on mobile attributes in the next few months.
This is not a drill.
For anyone who builds using responsive design there's unlikely to be any issue as (generally speaking) content is replicated across both mobile and desktop templates consistently.
Sites most likely to suffer are those who maintain a separate mobile site with reduced content and who are likely to show weaknesses from a technical SEO perspective where link equity doesn't flow from desktop to the mobile site.
The most substantial change will likely be that by having a mobile index, Google can run its ranking algorithm in a different fashion across “pure” mobile content rather than the current system that extracts data from desktop content to determine mobile rankings.