At least, that's what rumors say. Disney nor Marvel has not yet confirmed the former Buffy star's participation in the upcoming series. The rumors just started (and for good reason) due to Adrian Pasdar, who's voicing Iron Man in Disney XD's Ultimate Spiderman series, making the following statement:
“The funny thing is if people were to actually visit these animation rooms and see the guys and the girls that are doing it, with the exception of Eliza, who’s a stunning beauty, the rest of us, we don’t look quite as good as the animation characters they draw for us,” Pasdar said in a recent interview with IGN."
That's pretty much confirmation that Eliza Dushku's gonna be in the series, we're just marking it as a rumor because we don't trust TV execs.