Two Evo's at my house. Not sure how they get updated, but mine has the update and my wife's phone still does not. Have done all the updates on both at the same time.
I just got it last night. Was in the middle of updating Apps, market did one of it's nice little force closes, and when it opened it was the new market.
I was on the old Market till yesterday it changed. I loaded a custom ROM with the old market in it, it opened to the old market the first time then the next time I opened the new market so I guess once your account has been updated it will update every time you log in. Must be updating based on your email addy.
When I got the new market I tried searching for android market, after I typed android before I could type the rest my screen went white then the market restarted and it was the new one...
Not sure if my search (or attempt at one) did it or not...