A source has teased a new clue to Siliconera that the new Ultra Street Fighter IV character has been seen before in the comics. So either she's been in the Street Fighter II manga, or the UDON series.
Shoryuken speculates that it's Sarai Kurosawa, a friend of Ibuki who's made an appearance in an UDON comic, plus she's made cameos in Street Fighter III: 3rd Strike and Street Fighter X Tekken. Other potential ladies could be the Dolls, Gouken's daughter, and well a ton of other potential ladies.
My favorite theory at the moment comes from reader Reinhold Hoffmann who suggests it's just Yoshinori Ono dressed up as Chun Li.