In the original incarnation of the scene, Lucas said he had 3 squib set ups he had the time and money to shoot, and that all 3 times the squibs didn't go off at the right timing. He said in one shot, Greedo shot way before Han, in another shot, Han shot way before Greedo, and in the take they ended up using they shot at that same time, but you couldn't really tell. (this is from Lucas own words on the Making Movie Magic CD ROM). So he went in and made sure the dialogue made it clear that Greedo planned on killing Han. (you know, the Over my dead body... That's the idea). But I think the problem is a lot of people don't pay too much attention to subtitles, so a lot of kids got the idea that Han just shot Greedo in cold blood. I think he probably heard from enough kids who told him they loved Han the best because he killed Greedo in cold blood, that he felt an obligation to make his intentions more clear.

But either way I don't see how it changes anything about Han. Wether Greedo shot first or not doesn't negate the fact that he flat out told Han he planned on killing him.