That is unconfirmed. The reason I started this thread was because I saw that but couldn't verify the date. I can tell you this:
No other site claims March 31 as the release date.
AT&T (customer service phone line, chat specialists and stores) has no idea when the phone will be released. I couldn't even get them to admit it existed ... others have had different answers, but this is my personal experience.
Sony, via chat support, says the release date is Q2 ... I asked if there was something more specific, an actual date maybe, and they just said, "I don't understand, I told you the release date. It's Q2, sometime between April and the end of June.
That being said I've moved my eyes to other phones. If this gets released before I've picked something else, then maybe I'll still get it, but AT&T are ridiculous.