French site Xboxygen.com writes (Google translation):
"The Twitter Don Mattrick = fake Confirmed!
Some have questioned this account on Twitter DonMattrick since there was quite a big news in it (such as Final Fantasy XIV, an MMO etc).
It was virtually certain, but now we can confirm 100%: this account is not that Don Mattrick is Major Nelson himself has said. Now, you can stop monitoring this account unless you want to laugh a little?"
This is what Major Nelson had to say on his official twitter:
"..to see if the account belongs to a real person. If we had someone high up at Xbox twittering, you better believe they would have [the Verified Account badge] ;)"