Better late than never.
Recently CoroCoro released a picture of Arceus' official artwork, and a scan of it has been floating around Pokemon websites for a few days. Thankfully however, to coincide with CoroCoro's announcement the Pokemon Company has officially unveiled Arceus on their Japanese website and the 12th Movie website. Included was some nice quality Sugimori and Anime-style artwork. So, take a looksie. Thanks go to Bulbanews for collecting the images.
Also, since Pokecharms hasn't mentioned it yet and the news can be noted on the sites linked to above, an image showcasing the three Johto starters (Chikorita, Cyndaquil, and Totodile) and Heatran has been revealed for the 12th movie. All four are said to be included in the movie somehow, we just don't yet know to what extent.