We thought the Haggar billboard and Metro City license plate in the new car-smashing bonus stage would be the only Final Fight nods in Super Street Fighter IV. Scans from Famitsu show that Final Fight's two shirt-wearing protagonists, Cody and Guy, will appear as playable fighters in the game. The appearance of Adon, along with Cody's striped prison getup, means that the fighters are appearing in their Street Fighter Alpha incarnations. Well, those clues and the heading on the article saying that the three characters are from that series.
A sidebar in Famitsu shows Guy parrying a kunai, suggesting that Street Fighter III favorite Ibuki might be joining the fight as well. You remember Street Fighter III, right?