Edit: I see, Mashable got some quotes from Horowitz that aren't in the original Google+ post: http://mashable.com/2012/01/23/google-plus-allows-pseudonyms...
Google+ is not, however, accepting new pseudonyms. This is designed for “established ones.” Horowitz explained that the new account naming option is intended for “people who have earned credit in other social systems and want to redeem that credit in Google+”.
This is really important since you have to fill out a Google+ profile when you sign up for a Gmail account. I can't believe they would throw away the ability to make an anonymous email account.