I was in the comic book store the other day and found the Age of Ultron graphic novel. It's not an old storyline. The original comics came out in early 2013 which makes me wonder if the whole thing was written with the movie in mind. Then I have to slap down my own doubts when I see that the comic is not an Avengers comic. It spans much of the Marvel Universe. Major rewrites would have to be done since they can't use Spiderman, The Fantastic Four, or the X-Men in the Avengers movie. From what I've heard, they're even having to change who originally built Ultron from Hank Pym to Tony Stark since the Ant Man movie won't be coming out until after Avengers 2.
In the comics, Ultron is an AI created by Hank Pym (Ant Man) who various Marvel heroes, but mainly The Avengers, have been fighting since the late '60s. Destroyed again and again, he keeps coming back in new bodies. In this storyline, the body of a Spaceknight (not necessarily ROM's Spaceknights) is unearthed and Ultron wakes up inside it. He manages to elude the heroes. The comic then skips ahead to about six months after the Earth, the whole Earth, was attacked simultaneously and conquered in about eight and a half minutes. Most of the heroes are dead. Wolverine (X-Men) and Susan Storm Richards (Fantastic Four) go back in time and kill Hank Pym. When they return home they find that without Hank's help and without Ultron's creation, a robot Avenger named Vision, some key battles went poorly and the world is on the brink of being destroyed by the winner of the Asgard/Latveria (Thor/Dr Doom) War. So they have to go back and not-kill Hank Pym and figure out how to fix things.
As you can see, there's been very little mention of Hulk, Thor, Captain America, Iron Man, Hawkeye, Black Widow, Nick Fury, or Agent Coulson. In fact, it's been almost exclusively heroes yet to be introduced and heroes whose rights are in the hands of other studios. Our Avengers are there, just not major enough to be part of a one paragraph summary. So major rewrites were needed.
Honestly, this is not a must-read graphic novel. It's there for those unwilling to wait until 2015 for the movie. If you get it, you may also want to look for the Age of Ultron Companion. It collects other titles and tells their shorter stories about actions taken to fight back against the initial Ultron invasion.
note: It should be stated that the comics took place while Doctor Octopus's mind was still in the body of Spiderman. I didn't follow the storyline closely, but from what I've seen Doc Ock made for a good hero.