I usually stay quiet on possible production issues when a new camera is released. The likelihood that there will be some on a product as complex as the D4 or D800 is near 100%, especially given the way (the limited) field testing occurs these days. In essence, first purchasers will almost certainly find some issue that needs addressing. Historically, Nikon addresses such problems with real fixes. Maybe not instantly, but with reasonable haste.
Some online sources are now reporting issues that are known with loose journalistic standards. For example, Engadget wrote "a 'small number' of units can lock up and become unresponsive." That's actually incorrect, I believe. There appears to be a firmware bug in every D4 and D800 that, when you have Highlights and RGB histogram active (my usual recommendation and what most pros would have set), the D4 and D800 will experience random lockups. The quick fix is to turn those things off. The long term fix will likely be a firmware update. Nikon has acknowledged a problem, and I have no doubt that they'll act quickly to put together this and a few other "fixes" into a firmware update soon.
Then there's the usual "doesn't focus right" complaints. I say usual because as we've gotten higher resolution cameras we're finding more people who aren't handling their cameras right or understand the autofocus system correctly. We've had this complaint now about the D3/D300, the D7000, and now the D4/D800. Yet the vast majority of those complaints actually turn out to be user misunderstanding or AF Fine Tune tolerance differences. Yes, I'm aware of the "left side doesn't focus same as right side" complaints. I can't verify them on three bodies I've tried, so short of actually getting a body in hand that displays this problem, I can't really say anything. At the same time, I know that if you do experience a real issue and can report it to Nikon clearly and show examples, they'll definitely look at your problem and fix it if they find it is real. Ranting online won't fix your problem, should you actually have one. Sending your camera and maybe lens back to Nikon with a clear report of the problem and how it is triggered will.
Finally, let's put some numbers on things. At this point Nikon has produced at least 25,000 D800 bodies. A typical early run manufacturing problem rate would be 5-10%. That means that of the 25,000 bodies first shipped, we'd expect 1,250 to 2,500 of them to have an issue reported by the user, of which not all of those will turn out to be real manufacturing or QA problems (see previous paragraph). That's normal. Over time, problem rates go down.
This last week I've had to write the following many times to people who've emailed me: if you decide to live on the bleeding edge of technology, you'll sometimes get cut. That's why they call it the bleeding edge. If over 40 years you buy 10 new camera bodies right at introduction, the law of statistics say that probably one of those will have a real manufacturing problem. Likewise, complex software (e.g. camera firmware) never ships with no defects at version 1.0.
If you don't like those odds, don't buy the first product that comes off the assembly line, simple as that.
Moreover, here's the other thing I've had to write in email responses to a lot of people this week: never, ever rely upon new equipment until you've had a chance to thoroughly and completely vet it. If you sold your D3s in anticipation of getting a D4 and then ran out the next day after getting your D4 to do a for-hire job, any problems you encountered are due to your decision making, not Nikon's manufacturing or QA. You should have kept your D3s and did the job with that. You should be testing the new camera in parallel with your old, and only switching when you're convinced that it's production ready.
That last bit brings up a problem for Canon: just how soon before the Summer Olympics is the 1Dx going to actually appear? If I were photographing the Olympics for hire this year (unfortunately, not happening though I'd love to), I'd be vetting the equipment now. No way would I want to get a camera in July and head to the stadium within a few weeks after that. While CPS and NPS have service and loaner centers at the Olympics, I want to know that my gear is top notch and I understand it perfectly long before I rely upon it for a one-off event like the Olympics.