The company announced the policy change in a recent blog post. Knowing that not all of the old YouTube users have Google accounts, Software Engineer Brett Hobbs was careful to explain the benefits and knock out any misperceptions those users might have.
Most notably, he clarified that YouTube usernames won't change, they'll just be tied to Google accounts behind the scenes. He wrote that it will make things much easier for the YouTube team because sorting between the two account systems is a big pain, and that Google accounts have better security, so it ought to be a win for everybody.
You don't need to be a Gmail user to have a Google account; you can sign up with any e-mail address. Wary of a privacy backlash, Google has made it clear that your YouTube activity won't start automatically publishing in your Buzz feed once your accounts are linked. You'll have to opt in to that feature.