After hearing about Age of Ultron, I took some time to digest how this would work, with what we have seen thus far in the MCU, and the beginning of Phase 2 with IM3. Frankly, Ultron didn't seem to fit in the current MCU environment by my estimation.
But, then it hit me...there was something I hadn't factored in....
Agents of SHIELD.
I am theorizing that at some point during Agents of Shield, as we progress towards the release date for A:AoU, we'll see a story arc in Agents of SHIELD which will introduce us to a super intelligent scientist who SHIELD is keeping an eye on (queue the cameo appearance of whoever will be playing Antman in the movie) and in this story arc, we'll get a brief tutorial on what we need to know about our scientist and what he is doing, as well as see the development of Ultron's programming progress.
By going this route, Marvel could introduce Ultron, without needing Antman to release ahead of the Avengers sequel, and without even needing the Antman character in the movie, except in maybe a cameo or supporting role as himself, and not as Antman...and it can serve as a catalyst for what drives the scientist to become Antman.
It's an ideal way to not only boost interest and viewership of Agents of SHIELD, but also a perfect way to serve as a hype machine for the movie release, and it rewards those loyal viewers who not only go see the movies, but are supporting the television product as well.
It's a genius marketing move, if this is the route Marvel / Disney / ABC takes.