From the latest Entertainment Weekly:
GL: If you really look at it, there's hardly any changes at all. The thing that really caused the trouble on "Star Wars" (notice he doesn't call it "A New Hope"?) is the whole question of whether Han Solo or Greedo shoots first. The way it got cobbled together at the time, it came off that [Han] fired first. He didn't fire first.
EW: So you consider this a correction?
GL: It's a correction. [When I made "Star Wars"] I said,"Well, I don't have that shot, so I'll just, you know, fudge it editorially." In my mind [Greedo] shot first or at the same time. We like to think of [Han Solo] as a murderer because that's hip - I don't think that's a good thing for people.
Seriously, is he even aware of how silly this sounds? Did we all think it was "hip" to think Han was a "murderer"? From the dialog in the scene, it was OBVIOUS that Greedo intended to kill Han - it was never any worse than self-defense.
Lucas creates a "Western in Space", where people carry sidearms as a matter of course, then 20 years down the line convinces himself that having a classic "gunman" character use his sidearm in self-defense makes him a cold-blooded murderer. THEN, another 6-7 years down the line, he convinces himself that it was never supposed to be that way in the first place - he just "fudged it editorially".