In the comics, Ultron is a big bad robot hellbent on destroying humanity, but Joss tells Marvel LIVE that he'll be changing the villain's origins for the movie -- saying "We were crafting our own version of it where [Ultron's] own origin comes more directly from the Avengers we already know about."
Bolding mine, because I'm gonna take a stab in the dark and predict that J.A.R.V.I.S.is gonna go haywire and robo-jack one of Tony's suits, or secretly build his own. HEARD IT HERE FIRST, PEOPLE.