Shaymin’s sky form has been confirmed to exist via a printing company employee in Japan, who (we assume) saw it on a poster in his office. On his blog, he describes what he saw (only the parts relevant to Shaymin are translated below):

I was absent-minded at the office and carelessly glanced upon the sky form of Shaymin. / When I saw it, it was as shocking to me as when I first saw that Darkrai had legs. Its eyes are like Girafarig’s / it almost looks like a failed Eevee evolution, though it still looks good / it sort of looks like Delcatty too.

He also says that the land form of Shaymin is the regular Shaymin, meaning the trademarks last month for a “land form” and “sky form” Pokemon applied to Shaymin.

Thanks go to Unripe Ume Fruits (the employees’ blog) via AAPF (who found the blog) via Vincent (for translating some of the original blog post) for this news story’s information.