For a brief period yesterday, Nintendo listed ye olde Shovel Knight amiibo as being compatible for training in battle in Super Smash Bros. for Wii U, which seemed to suggest he is going to be a DLC character in the fighting game.
Yacht Club Games went on to deny that Shovel Knight is coming to Super Smash Bros., stating, "Appreciate the enthusiasm for the idea, but I believe this listing was made in error." However, there's still room for speculation. Yacht Club previously denied that Shovel Knight would release at retail after it was listed on a retailer's website, and we all know how that turned out.
While this seems like pretty good evidence, it bears keeping in mind that Nintendo at one point listed DuckTales: Remastered as being amiibo compatible -- which, you might know, is false -- so this could just be another unfortunate goof. Though I have a hunch that isn't the case.
The Shovel Knight amiibo is listed as releasing holiday 2015, which is sooner than originally expected.