While examining closely the Hulk's™ Helicarrier Breakout Lego set on the Lego website, a likeness of none other than Ultron appears on the laptop. I originally assumed this image to be a scan of Loki's spear right next to it, but on closer examination it does resemble the 'living automaton'.
This could of course be wishful thinking, but it would make some kind of sense to use Ultron as an easter egg for the fans as possibly the mainframe of SHIELD's computer network, thus setting up future movies. (Having a sentient AI as a mainframe for a computer network in control of high-tech weaponry - what could possibly go wrong? Oh yeah quite a bit - see The Terminator series.)
Ultron was used as part of the SHIELD set up in the Avengers EMH cartoon series