I've been an 34-xbr800 owner for about 9 months, and have had a chance to view the new 910 side-by-side with the 800 at circuit city. Both sets were running the in-store HDTV feed, connected with component cables. I'll try to answer the question many xbr800 owners are asking: "did I make a mistake in purchasing the 800? Is my TV now inferior?"
Well, I wouldn't say inferior, but the degree to which the 910 looks better than the 800 is directly correlated to your viewing distance. At 5 feet, you'll definitely see the 910's super fine pitch screen is better. Solid areas of color look...solid. On the 800, you'll see the vertical stripes of color with black in between, not solid.
However getting out around 8 feet, the difference really drops off. I did a good 15 minutes of continuous viewing at 8.5 feet, which is the viewing distance for my 800 at home. Here, you really had to scrutinize to see the difference in picture. Yes, the 910 was still better, but only slightly so. Also, I didn't think the light output difference was as drastic as some people have described it. Using the default standard, movie, and pro picture modes, the 910 was only moderately less bright.
So there you have it. If I was viewing my 800 at a 5-6 foot distance, I'd start looking for people to sell it to, and get the 910!
On a practical note, the 910 has one negative that the 800 does not. The top of the 910 is not flat, it slopes down towards the front of the set a little. This would make a less stable base for a center channel speaker. The 800 has a completely flat surface on top.
I'm going downstairs now to enjoy my 800, and remind myself I need to be satisfied with a $2000 television set!