Here's some speculation that sort of makes sense: we just reported that the AT&T Venue Pro is available to purchase again direct from Dell. What's odd (and what we missed) was the expected ship date: 8/30
That's a long time off for a phone that has been out for awhile. Considering that we just got word of "Mango" being RTM'd and sent to carriers, could this 6 week delay be due to Dell putting Mango on board? Dell did just pull the Streak to load some new update, so it's not unprecedented and AT&T is pretty aggressive in the Windows Phone area.
We've heard that Toshiba-Fujitsu are launching a Mango device in August, so why not Dell? Then again, this is Dell, so lets not put it past them that this is just NoDo.
Source: Dell; (Thanks for catching this, @wp7ap)