Microsoft is making massive progress with it's next big OS update -- Mango -- for Windows Phone 7, with the company recently releasing the beta to registered developers and enabling anyone with a developer account and a registered phone to update to the beta bits.
One caveat with the Mango beta was that Twitter wasn't quite ready yet (apparently because of a dispute between Microsoft and Twitter) and is locked down, but it looks like Microsoft is testing a new build of Mango with Twitter functionality working.
WPCentral reports that a few users on Twitter have begun tweeting with the client label "via Windows Phone (via Windows Live)." Previously Microsoft had tested the functionality where the client label read "via Windows Phone (via Windows Live)INT." Neowin has heard from multiple sources that the reason Twitter functionality is yet to be shown off in public is due to tension between Microsoft and Twitter, and that the functionality had been very "hit and miss" before the Mango beta and was not worth including yet.
One tweet is from the Microsoft Windows Phone 7 official twitter account, and another two from Ken Dacey and Christian K who are Windows Phone engineers. The Windows Phone 7 account was quizzed by a user as to if this was a new build, which they would not comment on.
It's not clear if developers will get another beta build before release, but it seems unlikely considering the rumored September release month.