We just stumbled upon some footage of a yet unannounced Nokia Windows Phone smartphone. The device that appeared on a promo video created by a Nokia developer is believed to be the Nokia 900 Ace, which leaked right before this year's Nokia World, but didn't make an official appearance at the event itself.The smartphone bears some resemblance to the Symbian-running Nokia 700, but features a much larger screen. According to some speculations, based on comparing the icons sized in this video and in the similar Nokia Lumia 800 promo clip, the WP device you are seeing here has a screen diagonal of at least 4 inches.Of course, this could turn out to be just a reference picture and not an actual device, so don't get too excited just yet. However, it would certainly make sense for Nokia to have another interesting WP smartphone to launch at the start of next year. The Lumia couple is certainly nice, but it won't be able to take the company out of the swamp it has put itself into alone.