Besides the issue of the regular version being a combo disc, is there a difference between this and the version that comes packaged with the A3?

It has disc art and I think it loses one extra feature that was present on the SD side. There is also no UPC on the back of the case.

The disc is not the HD-DVD combo format, just a straight HD-DVD. I sold mine to a friend since I already have it on BD.

Oddly enough, I found a copy of this on the sales floor at CC. No price tags on it and it had a sticker that said "Not For Sale". Wonder how it ended up there?

The non-combo version doesn't have the audio commentary. It's only on the SD side.

I'm guessing it came from an A3 they opened up and are using as a demo machine.

No, it was wrapped and sitting in the HD DVD section with all the other combo releases of 300. I wonder what they would say if I brought it up to the front?

Just like the poster said above, it was probably from their A3 demo machine they opened and it made its way out on the sales floor. Both movies inside the box are wrapped just like normal HD DVDs.