With all the hype surrounding the latest PS4 and the announcement which took place last week in New York, Microsoft have indeed reacted to this, although not in the way that you may have expected.
Of course there is, and probably will always be, a dispute between Sony and Microsoft who are the biggest competitors when it comes to Video Game Platforms, WHOIS data, has allegedly stated that XboxEvent.com was registered yesterday.
Another rumor is that Xbox may reveal the Xbox720 at E3 on 11th June, however with the Game Developers conference happening very soon perhaps Microsoft may decide to announce then.
More will be released on this story soon!
Updated 17th April 2013: Please note that since this post has been published the website mentioned is now removed. Please visit https://pwnersmanual.wordpress.com/2013/04/09/xbox-720-reveal-date/ for more Xbox 720 news.