It's a silly upgrade for Nikon. Why?
It has marginal megapixel increase over the massively cheaper EOS 5DII, though it does have the build/body of the D3/1D series. However, is that really going to be worth thousands? Probably not. Same goes for the 1Ds, but at least the 1Ds price isn't as high. Hell, the Sony has the same sensor (it seems), and costs way less.
Um, the 1DsIII came out at the same price point. This isn't a 5D competitor, that'd be a D700x if one is released. The camera competes directly with the 1DsIII, and it's priced and featured accordingly. The sensor base is the same, but it appears that the Sony's native performance didn't meet Nikon's standards, compare the noise characteristics once official samples are out and I doubt anyone would pick the Sony images as better... plus you don't have anywhere near the lens range with Sony at the moment. I'd not only lose 25% of my reach at the long end, I'd lose way more than the difference between an A900 and a D3x if I had to switch all my glass to Sony.
The base specifications are roughly the same as the 1Ds3 and processing power is barely increased over the D3 (in terms of megabytes/sec), despite there being a significant time gap between the releases. Where's the progress? The ISO range is lower than the D3, which is a step backwards.
Of *course* the ISO range is lower, you get low noise or high resolution, it's the same physics for the same generation of sensor. The progress is in resolution, not everyone wants the uber-high ISO. Many folks need the resolution- this is not a general purpose camera, it's not aimed at the prosumer market- if you don't need it specifically then Nikon doesn't expect you to buy it.
As far as performance, the D3 is a sports shooter's camera, this isn't. Why anyone would expect it to outperform a sports camera is puzzling.
Are all of you such bad shooters that you can't time a shot and need to machine-gun through a zillion frames to get what you expect? I shot battling Eagles last weekend, and I got great shots without holding down the shutter button for 3-4 seconds- I can't imagine many situations that are as dynamic as two Eagles locked talon to talon spiraling through the air- what are you all shooting that needs maniacal frame rates?
I was hoping for something better - say the same megapixels, but able to shoot at 7-8fps and a 12-15mpx DX mode. Cheaper too, the price is just insane. The D3x is equalising with the 1Ds3 with maybe a handful of very minor improvements - but a year and a bit later. I'm sure Canon will release the IV sometime, and yet again go "one better" (as is the way with cameras...).
The DX crop is a function of area against the overall resolution- you can't get the same megapixels and a higher DX crop. If you own Nikon glass and you need to compete where the D3x is placed, then it's a very welcome body.
Nikon shooters are better off with a D700 for full frame, and a D3 if they really want. If they're willing to hop to Canon, they can pick up a 5DII and a 1D3. All of this for less, and with the advantages of double bodies. Or even just pick one and spend the rest on lenses.
Again, it depends on what you're shooting and for what reasons. Blanket statements on professional tools are idiotic- a D700 doesn't give the same resolution, and if your client needs a higher resolution image, then guess what? Nikon didn't stop making the D3 when they started making the D700, and the same argument applies there...
As for "hopping" between manufacturers, that's a sure way to lose your ROI numbers. Depending on the job, you can sometimes justify adding a system, but switching when you've got tens of thousands tied up in glass, brackets and other equipment is almost always a losing proposition, in fact it's normally stupid[1.]
For me, the ability to crop makes the D3x much, much more attractive than the D3- though I expect that with the global economy Nikon will start rebating sooner rather than later at this price point, and I'll wait and see.
I've got an advertising shoot tomorrow afternoon, and I can hands-down say a D3 or D700 wouldn't give me one iota of advantage over my D2x, but a D3x would.
If I did a lot more frequent advertising work, I'd be in line even at the current price- because the dual CF card and high-resolution combination make total sense from a business perspective (I've been waiting for a week for the client to set up for the shoot, losing a shot to a bad CF card costs, tying up people, products and space for a re-shoot costs.) Shooting on multiple cards without having to stop and switch cards and remember to switch back is a very good feature in terms of the time to shoot and the "flow" of the shoot.
A lot of people made the "Why would you pay $2500 more for a D2x when you could get a D200?" argument. I tried that argument and my D200 didn't last 30 days before it got traded in (with a pile o' money) for a D2x.
If you don't need the difference, you likely won't understand- because it's not a law of averages type issue, like with most tools you pay a hyper-premium for the last 5% performance difference. You can buy a $5 hammer every 5 years, or a $40 hammer once- but only the craftsman can say which path is right for them.
Canon hasn't stopped making the 1DsIII because the 5DII is out- and they're not likely to- because some photographers need the differences, and Canon is going to cater to that crowd (who'll pay a premium because they're looking for a niche product.) You can make the same argument about the 1DsIII vs 5DII- and the same people will tell you the same thing- if they need the more expensive tool, that's the one they'll get.
A pro body is good for ~5 years- a $600/year premium for one body is hardly worth switching systems for even if we assume the list price is going to hold for six months. Follow that by the fact that you have no idea what the next Sony or Canon body you'd get in 5 years is going to be priced like and all the hullabaloo over price is really silly.
Here's my prediction: The D3x will come down in price just like the 1DsIII did- but more quickly. Folks who need it to compete now will pay the extra new camera premium and be glad to have it. Folks who think it will give them a slight long-term advantage will wait for the price to come down some. Folks who think they (or worse-yet other people) should switch systems because of a price point for a single body will lose a lot of money if they do, but probably won't do anything because they're mostly not the doing type, or they're more interested in numbers than photographs so they'll just waste money going back and forth like the high-ISO folks keep doing[2.]
[1] There are times when you need a feature set or need to get out of a manufacturer who's going in a direction that doesn't suit you, but for the price of a single body it's almost always about as dumb as you can get.
[2] There are folks who absolutely need high ISO performance, and there are folks who need to learn to expose properly- DPR is full of the latter as far as I can tell, the former can make an ROI case immediately for a switch that costs ~$5-10,000- the latter can just complain when the next manufacture leaps into the lead.