It's a rumour, and not officially confirmed (though Jay's tweet might be seen as confirmation), and I would not take this for granted until an official statement from Blizzard themselves (which will be very soon - possibly monday).
One day after my birthday! wooooo! I'll have a big hangover though
if this IS false i am sad
If true i'll be over the moon! (y)
Blizz is gonna troll us and push it too april 18....
It seems kind of legit.
The fact they haven't openly denied it is a good thing and are also aware that we are aware of it. I'm not sure if that will wind up being the exact date, but it at least gives us a good date to estimate around.