I've been doing some resarch on this on what is available on Sealife's website, and there appears to be an error or two in the site so I am taking my best guess here on a couple things.
Let's compare directly some of the differences, according to the spec pages (the DC1000 is listed first). My comments are added after each line, and keep in mind two things... I am a complete NOOB at underwater photography but have been a land shooter for 30 years. Check out my website listed below to see if I am all stuffed or what.
Differences:
10 megapixel instead of 8 (who cares... two megapixels don't make any practical difference in the real world, and stuffing more resolution on a tiny chip generally means higher noise).
5x zoom but longer focal length 6.3mm-31 vs 4x zoom 4.6-18.4mm (I assume this is correct, the website has an error on the DC1000 page calling it 4X in one spot and 5X in another... but the math shows 5X... personally, I see this as a good thing, because you can now use a wide angle adapter to increase your field of view)
Focus modes normal, Macro, landscape (infinity) vs auto, macro, landscape (infinity) and face detection in the 800 (did they remove face detection on the DC1000 or did they just not list it? Either way, I couldn't care less... face detection on an underwater camera, expecially one that is locked on infinity focus, is meaningless)
More narrow field of view on land (56 degrees vs 64.3 degrees) but almost identical in water 42 degrees vs 43 degrees. (I am guessing that's because of the case? I have no idea, would love to hear Joe tell us why on that one).
Flash... Auto, Force ON, OFF, Red-Eye reduction, Slow Sync, Pre Flash vs. Auto, Fill, Off, Red-Eye reduction (this is all good, especially slow sync).
Picture modes listed: Sea mode, Snorkle mode, Ext Flash Auto and Ext Flash manual vs. ext flash auto, external flash manual and sea mode. (The snorkle mode is apparently a different preset white balance... choice is good, this is a good improvement).
Land modes divver as well: Auto, Sports, Panorama, Anti-shake (land), Splash-water, Flow-water, Night Scene, Landscape, Aperture priority (Av), Shutter Speed Priority (Tv), Fireworks. vs. Auto[Program AE], Sports, Anti-shake, Sunset, Sunrise, Splash-water, Flow-water, Night Scene, Portrait, Landscape, Panorama, Snow, Beach, Pets, Av, Tv, User Setting, Candlelight, Text, Fireworks, Backlight. (I don't care a bit about the land modes, would gladly trade the extra snorkle mode for most land mode presets).
Shooting modes: SPY mode, Single shot, Continuous (3pics), AEB (bracketing), Continuous Flash, Burst, Pre-Shot, Couple Shot, Album Shot LCD 2.7" color LTPS TFT LCD vs. Single, Continuous(3pics), SPY mode(unlimited), AEB (bracketing) (This is all good as well, nice upgrade here).
Pictbridge not mentioned on DC1000 and yes on DC800 (who cares? I can't imagine why anybody would use that anyway with an underwater camera)
Software doesn't mention anything on DC1000 vs. Media One Gallery and Cool 360 SE International (again, who cares... first off, I am a Mac guy and none of that stuff works on my mac, and secondly the software was complete junk anyway.... free Picasa is MUCH better than Media One Gallery and Cool 360... well, guys, 1992 is calling and they want their software back).
To me, the biggest change is the lens... a longer lens is both good and bad, since it backs you up a bit (less light hitting the subject from the flash) and it gives a much narrower field of focus (at least out of water... I am still curious how that one degree difference on the FOV between these to lenses work in water). The WA lens is a nice addition though.
Love to hear your comments Sealife Joe.