From the official blog: "We'll start by launching a "You may know these people" suggestions box on the homepage over the next few weeks that shows you the YouTube channels of people who might be your acquaintances, and lets you easily connect and subscribe to them."
YouTube will choose your friends-to-be by using the information they already have: if you've sent a video to someone's email address (while being logged into YouTube), that person is a potential friend. Also, if you have your YouTube account linked to your Gmail account, Google can see which of your contacts have YouTube channels.
Of course, not everyone likes this idea; on Facebook and MySpace, you're meant to be able to connect with friends, but on YouTube some users would like to remain incognito. In that case, you can log into your YouTube account, open the "Privacy" section, and uncheck the "Let others find my channel on YouTube if they have my email address" option.