It's no surprise that Windows Phone "Mango" is getting Twitter in the fall. The feature is currently disabled in the beta builds given out to developers though, so it's fun to track its development along the Microsfot pipeline. What we can figure is this: a few Microsoft devs have been using it on their phones since May in a limited capacity, but recently new users have been given later builds of Mango with the feature enabled (see above image). Even Joe Belfiore is now using it.
In the last few days a few more folks on Twitter have started tweeting with "via Windows Phone (via Windows Live)" as the tweet source. Clicking the link redirects you to Microsoft's Windows Phone site. This was similar to another build from April where it said "via Windows Phone(via Windows Live)INT". (The difference there is of course "INT" which probably stood for "internal", as in development purposes).
One of the tweets comes from Ken Dacey of Microsoft, who was also the one responsible for the earlier "INT" Twitter posts a few months ago. He's been using Twitter on his phone since May, but another from @MSWindowsPhone only appeared in the last day (they won't confirm if it's a newer build, however).
Either way, Microsoft is making progress as expected. Hopefully registered AppHub devs will get a new beta build too.