Late last night, Valve dropped a small patch for Counter-Strike: Global Offensive. However, the patch notes don't mention a significant addition to the game files that hints at 'Prime Account Matchmaking'. Don't worry, it's not a new premium service.
From the SteamDB Github account: "CS:GO is running an experiment to find out whether players will have a better matchmaking experience when they are matched with players who are using a phone-linked CS:GO account.
"To join the experiment, you'll need to upgrade your CS:GO account to Prime status. Just click the UPGRADE button below, which will bind your Steam phone number to your CS:GO account (provided it qualifies, see the F.A.Q. below).
"If you own multiple CS:GO accounts, be sure to upgrade your favorite one since you can only upgrade one CS:GO account to Prime status with your qualifying phone number.
"Once there are enough Prime accounts, we will begin Prime Account Matchmaking and will start prioritizing matching Prime status players with each other."
Players are widely elated: in theory, Prime matchmaking will seal off honest players who play through a single account from the likes of smurfs and cheaters, who commonly switch accounts as old ones get banned or rank up. It's unlikely to be foolproof—someone somewhere will be prepping a crate of SIM cards as I write—but there are precautions such as the exclusion of VoIP numbers to make the system more robust.
A screengrab from /u/lolBEEF purports to show the beta up and running, at least for some: