Panasonic announced the Lumix G1, a smaller Micro 4/3 quasi-DSLR and Sony announced the 24mp A900, as expected, effectively bracketing Nikon DSLRs at both the bottom and top and generating a lot of email to me while I was out of Internet access ;~). The specs of the 5DII put it in the same bracketing category as the A900. So I guess I need to comment.
First, congratulations to both companies on their efforts. The G1 looks promising as a replacement for high-end compact cameras, perhaps finally delivering on the true 4/3 promise. Putting 12mp across 18mm instead of the 5mm to 7mm in the better compact cameras is probably enough to make this a high-end compact killer if everything else is up to snuff. I'm not enthused about the lenses, though. The bigger the sensor, the bigger the lenses get. The 28-90mm equivalent zoom that was announced with the camera doesn't collapse into the camera like on compacts (the price of having interchangeable lenses), and pushes the camera beyond shirt-pocketable. Give me three small pancake primes (only one has been announced) and things start to look much better. Still, I think it'll be worth exploring as the low-weight hiking alternative I've been looking for.
Meanwhile the A900 becomes the megapixel bargain (at least for a short period of time). Ergonomically, it feels just a little clunky, but it appears that someone in the Minolta-KonicaMinolta-Sony design chain is starting to reign in the scattered placement of controls: most things can be done with one hand, though you'll move your finger off the shutter release to do so (a Canon fault, too). Initial image quality seems to put the pixels at least at the D300 level, which means that the A900 does indeed become a step up in what you can produce with large prints from the now-ubiquitous 10/12mp cameras.
Second, don't expect any such bracketing maneuvers to last long. Every camera maker is currently poking at "gaps" in the other makers' lineups (witness the D90's video capability). But history has shown that any such successful poke is matched within a full product cycle generation. At best, you get half-cycle advantages. In the Micro 4/3 arena, I suspect we'll see a Coolpix APS response in 2009 and we already know that Samsung is preparing to enter that realm, too. In the high megapixel count arena, it appears that Nikon's response is currently targeted for early 2009 (WPPI and PMA), though there's still an outside chance that it could be preannounced/leaked as early as Photokina. I expect that high megapixel camera to be highly modular and unique at the time of its launch (more gap poking). I should point out that nothing precludes Nikon from backfilling a D800 (24mp Sony sensor in D700 body) or a D3x (same sensor in a D3 body) at any time should Nikon decide that the A900/5DII needs a direct response.
I do think it curious that Nikon didn't make an immediate response, though. The A900 development and likely price has been well known for 18 months and the sensor is available to license. While the Canon 5DII specs have only floated up recently, it was pretty much a given that it would have to go upscale from the original version. So one can only conclude that Nikon thinks they have a better solution again. I've now noted seven different larger-than-APS prototypes they've tested (including what became the D3), so Nikon has been busy trying a lot of different options.
The real question all the camera companies are dealing with is how things will eventually settle out for serious cameras. It's looking more and more like the low-end (e.g., the consumer-driven volume business) will slide towards all-in-one (video and still) EVIL APS cameras instead of traditional DLSRs. This puts Canon, Panasonic, Sony, and perhaps Samsung in strong positions to win large unit volume shares. Historically, Nikon has not done well at true consumer cameras, so it will be a real challenge for them to hold onto the substantive gains they made with the D70/D50/D70s/D80/D40/D40x/D60/D90 progression. For serious still photography it's looking more and more like the old film market: solid FX DSLRs and MF cameras are the answer. That leaves tweener products like the Nikon D90 somewhat endangered long-term, I think. Certainly not this year or next. But long-term those products become either the new "entry level serious" or go away completely.