This may have been posted before, but I was watching the "Making Magic" CD-ROM which was used as a sort of primer on the SEs, and I found what may be Lucas's definitive comments about the Han/Greedo scene. This is in contrast to the usual line that Lucas uses, that he had always meant for Greedo and Han to shoot at the same time. My view of the quote is that Lucas probably got so sick of explaining it that he just uses his current line to simplify things.

Lucas, circa 1997:

"Well, the change in the Greedo scene: it's always been a little difficult exactly how Han shoots Greedo and what Greedo does to provoke that. And obviously, the situation was that Geedo's gonna shoot Han, Han outdraws him and kills him. But in the film, it sort of appeared that Han shot him without Greedo knowing what was going on. That always bothered me so we added a new shot - a wider shot - where you see Greedo fire his gun and have it richochet off the wall, and then Han shoots him so it doesn't look like Han shot him in cold blood. We did have some difficulty in the timing of the laser so that it looks believable that Greedo could have fired the gun and then Han would have fired shortly thereafter but still have it far enough apart for it to read, for an audience to see that there were two separate gun shots going on."