UPDATE: It would seem the Amiibo is legitimate.
http://www.nintendoworldreport.com/rumor/41040/shovel-knight-digging-into-super-smash-bros-wii-u3ds
UPDATE 12:20pm ET August 27: The Brighton branch of UK retailer GAME posted and deleted shortly thereafter a tweet announcing Shovel Knight Amiibo preorders would open tomorrow. The Amiibo was said to work "in future Yacht Club Games products for Nintendo consoles" and was part of a Shovel Knight series, but the art was not final. Original story follows.
A rumor has been posted by Nintendo Life and backed up with several sources indicating Shovel Knight may be about to dig into the Smash Bros roster.
According to the video below, several sources have been advised that the Yacht Club Games character will join the roster, with an announcement perhaps as soon as tomorrow night at the Indie MIX event at PAX. A Spanish retailer has posted a mockup of a Shovel Knight Amiibo, though it does use fanart for the box's backing.
It is not known if this is a result of the Smash Ballot or if a separate deal was made, however the ballot ending in October leans it toward the latter.