Look, it's happening, OK? From Kotaku:
A GameStop employee has sent us cell pics of the internal announcement for the coming Xbox 360 Pro price drop announcement. They also included pics of the shelf art all of which will go on display on July 13, the day before E3 kicks off with Microsoft's press conference.
So, while still not official, it seem pretty clear that come Sunday you'll be able to pick up an Xbox 360 Pro (aka Premium) for a penny shy of $300.
More through there, including pictures and a man shouting, "IT'S HAPPENING."