Via GoogleBlog:

But all that could change. In 2012, an organization called the Internet Engineering Task Force (IETF) created a new email standard that supports addresses with non-Latin and accented Latin characters (e.g. 武＠メール.グーグル). In order for this standard to become a reality, every email provider and every website that asks you for your email address must adopt it. That’s obviously a tough hill to climb. The technology is there, but someone has to take the first step.

Today we’re ready to be that someone. Starting now, Gmail (and shortly, Calendar) will recognize addresses that contain accented or non-Latin characters. This means Gmail users can send emails to, and receive emails from, people who have these characters in their email addresses. Of course, this is just a first step and there’s still a ways to go. In the future, we want to make it possible for you to use them to create Gmail accounts.