Shaymin's full Sky Forme image has been revealed on metal Pokemon badges from a Bandai Pokemon "Can Badge Collection," as you can see on two of the pins below. Shaymin looks like a reindeer crossed with a dog, has four legs, and bears what appear to be red wings (that look similar to poinsettia leaves). Will Shaymin replace Delibird as the Pokemon Christmas mascot?
The Shaymin, Mamoswine, and Giratina badge below will be released in July - originally, the set was advertised as containing a "New Pokemon," which is obviously Shaymin Sky Forme. Could one of the "New Pokemon" referred to on other products be Shaymin Sky Forme as well?
Speculation: Those red leaves look vaguely similar to the red slash in the movie logo. Could they be the same thing?
Small note: Although the new Pokemon resembles Shaymin and could perhaps be its new form, it is never actually labeled "Sky Forme" anywhere. It could be a new Pokemon all together, though this is doubtful because of its strong resemblances to the original Shaymin. You never know, though...