The hot new Microsoft rumor is that when the new Xbox Experience launches there will be a five hour launch event on September 25th.  Not only will is be an event to show off the new user interface but it will it is rumored to be streamed live to Xbox users as well as be broadcast on G4TV.  The five hours aren't just going to be spent showing off the new UI though.  The rumors are that there will be new game announcements (Halo 4?), new Xbox features (music store and Sirius channel) and highlighting the GTA IV expansion pack exclusive for the Xbox 360.    The exclusive games are rumored to be Mass Effect 2, Assassins Creed 2, a new Rare built FPS and Halo 3 DLC.  

In most industries rumors are exactly that, rumors.  The thing about Microsoft is they tend to leak their own rumors to help build hype within their hardcore base.  I would not be surprised if we saw the 360's motion controller announced with several games supporting the device.  I expect we will hear what Bungie has been working on as well since their game wasn't announced at E3 or at PAX.  This announcement could be very huge for Microsoft as it very well change people's minds about the 360.  If Microsoft can essentially create an HD version of the Wii that cost less than the Wii they very well might have a chance to compete with the Wii.

Source:
xbox360fanboy.com
giantbomb.com