Posting from Hollywood: New details have emerged about an Xbox-branded Microsoft tablet in advance of a mystery news conference planned by the company for this afternoon.

Microsoft news blog LiveSide.net has details on a Microsoft tablet called Xbox Surface, some of them via the site Shifted2U.com. They come in the form of a very legitimate-looking spec sheet — revealing a 7-inch screen with a 1280 x 720 resolution, SD card support, and a speedy 3.1 GHz processor, which sounds like it would be more of a battery hog than the chips used in existing tablet computers.

Mary Jo Foley notes that Xbox Surface could be linked to the Xbox SmartGlass project, unveiled at E3, a plan by Microsoft to connect Xbox Live to apps on Android, iPhone and Windows Phone. As we reported in the past, the trademark filing for SmartGlass referred to both hardware and software.

Whether this is the tablet that Microsoft is revealing this afternoon isn’t clear. At this point we don’t know officially if Microsoft is actually revealing a new tablet today. But we won’t have to wait long now to find out.

After traveling from Seattle this morning, I hitched a ride from the airport with Wilson Rothman of msnbc.com and we’ve made our way L.A. to the site of Microsoft’s mystery event in Hollywood this afternoon, at Milk Studios.

An initial drive-by of the location didn’t reveal any secrets. Crews are busy loading audio and staging equipment through the rear entrance.

Stay tuned for live coverage from the event this afternoon. The doors open at 3:30 p.m., but no word yet on when the event actually gets going.

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