Oh gosh, someone here in one of these threads (perhaps even a few pages back in this one) posted an amazingly complete list of the audio differences between the original '77 stereo mix and the one used for the new DVD. I'll try to find it. It was actually a link to an audio file with one channel being the '77 mix and the other the current mix, and there were time codes for each audio difference. Amazing.

Anyways, most of the differences were "sweeteners" to explosion sounds or additional droid sounds. The four that stand out to me are:

1) The whooshing sounds of Alderaan asteroid debris as they stream past the Falcon cockpit window.

2) The glass explosion sounds when blaster fire hits any surveillance camera during the shootout in the cellblock.

As has been demonstrated to me by the local experts, those differences are legitimate. The other two (listed below) dialogue bits were not in the 1977 stereo mix, but rather were featured in the '77 mono mix:

a) C3PO's detailed tractor beam speech

b) a Stormtrooper's "Close the blast doors."

So, it turns out there's only two really audible (to me) changes, and a bunch of minor audio changes that I would never notice if they were not pointed out to me (I'll try to find the post about that audio comparison, and link to it). And, as far as I know, there are no visual differences (it being convincingly demonstated to me that the opening crawl is the real deal).

Edited to add : There's a lot of information in this thread which I started to address this very issue. I'm 98% sure the post I'm thinking of is somewhere in there ... but, in any event, the whole thread is about what's '77 and what's not in the new DVD of Star Wars.