As Dark Horizons notes, Kick Ass didn't exactly set the box office world on fire, which is pretty much the key thing that would sell a sequel. Even director Matthew Vaughn has moved on to new projects (most likely X-Men: First Class) during the time that Millar says the movie will film. So, why is Millar saying Kick Ass 2 is going to happen? Because he's got a new magazine to promote -- an irreverent pop culture magazine called CLiNT that will also feature brand-new serialized Kick Ass stories. The man knows how to draw attention to himself.
Is a Kick Ass sequel completely out of the question? Probably so, unfortunately. Studios, now more than ever before, don't seem interested in anything they perceive as a financial risk. So, is Mark Millar nuts? Maybe. It won't stop me from reading his comics though.