T-Mobile and Nokia announce December 14th event, Windows Phone cometh?

Big news from Bellevue, as T-Mobile and Nokia have just announced that a special event will be held the evening of December 14th, promising something "special in the works." The companies were too smart for us this time, choosing not to reveal any code names via the image file; however, the tiled design of the invite seems to be a rather strong indication that Windows Phone is in the mix here. Could it be an existing Lumia device, or will it be something completely new? We have precisely one week to mull over the prospects.