This is legit, he really is on Google+. I never said he was stupid, just an asshole.
In case anyone is still doubting that it is the real Zuck on there, Scoble tells me that Zuckerberg indeed meant Google+ account when he referred to Google account. But the real question is, why are people so surprised that Zuckerberg would chose to be on Google+?
Perhaps the answer lies in the precedent set by Google founders Larry Page and Sergey Brin, who seem to have shied away from interacting on Facebook as themselves. (According to Steven Levy, Brin is actually on Facebook as a pseudonym. Google Chairman Eric Schmidt is also rumored to be on the service, independently of when Mike impersonated him).
Page’s and Brin’s behavior aside, plenty of other founders (Myspace Tom for example) have shown that it’s perfectly normal to partake and enjoy competitive services, and that it shouldn’t necessarily be considered an act of espionage. I for one just hope Zuckerberg is more prolific on Google+ than he is on Twitter.