You'll be able to get it at http://www.t-mobile.com/wmupgrade/, SlashGear has the full text of the press release:
T-Mobile USA, Inc. today announced that T-Mobile Dash™ customers will be the first in the U.S. to download and experience the benefits of the new Microsoft Windows Mobile 6 software. Starting tomorrow, May 4,
T-Mobile Dash customers can upgrade from Windows Mobile 5.0 to Windows Mobile 6 via a free desktop download available directly from T-Mobile.
Not much more to say, eh? Well, it does look, from the leaked version, that T-Mo has included at least a couple of custom apps in there beyond the standard WM6 fare: the MyFave's app (which is strangely necessary for their "my favorite 5 friends are free" plan) and audio controls on the today screen. I'm one of the cautious who's waiting for the official update to get WM6 on my Dash. Expect a review of WM6 on the Dash on Monday.