It looks like T-Mobile will officially release the Dash Windows Mobile 6 Standard upgrade on May 4th. I expect that you will be able to find it on the T-Mobile USA website on that date.
But if you are impatient and cannot wait a week, a post on Howard Forums has leaked the upgrade. It is apparently the official upgrade and a few people brave enough to install it have all reported no issues with the upgrade.
A word of warning: You are on your own if you install this leaked upgrade and run into issues. And remember that as with any ROM update, all your programs and data will be wiped out. Don’t forget to do a data backup first!