Now that I've had a chance to look through all that Nikon has been doing worldwide in terms of promotions during the past month and check some sales numbers, I've come to a conclusion: Nikon was in danger of not meeting its DSLR body shipment estimates for their fiscal year (which ends on March 31st). (Note I wrote "was." Still, we won't know if they were successful for another 45 days or so when they report results.)
Consider the US instant rebates (remember, the US is just over a third of Nikon's sales). Note that they all have a body requirement in them. Note further that NikonUSA really doesn't care which body you buy. I've even heard reports that some Nikon reps are suggesting to their dealers to just sell a D3000 with an expensive 70-200mm lens, then buy back the D3000 at some discount and sell it as used. The 70-200mm purchaser still gets a discount (though not US$400), some lucky future purchaser will find that they can get a very cheap D3000 that's essentially unused and warrantied (Nikon wouldn't be able to trace the faux sale). Nikon wants camera bodies off their inventory, that much should be very clear.
If you look at the most recent sales numbers of sell through at retail that are available, you see some indicators as to why: the D3000 and D5000 are not holding up the lower end volume. In particular, the D3000 doesn't seem to be holding the volume that the D40x and D60 established. The D300s is weaker than the D300 was. The D90 is finally showing signs of slowing down (it should, it's coming up on two years old).
So why not just discount bodies (as with the cash back offers in the UK), or perhaps extend dealers better terms if they order more inventory? I'll deal with the latter first: the danger of loading the channels with inventory this late in Nikon's fiscal year is that this spills over into a lack of orders in the first fiscal quarter of next year. It's a classic tech inventory problem: stuffing the channel generally only works to improve one quarter's numbers (unless, of course, you do the not-so-legal stuff-and-buy-back repeatedly). You'd only do that if you knew that you had new products that would help you make your next quarter's numbers. (Yes, you can read between the lines that I'm guessing that there aren't any new blockbuster Nikon products that'll ship in enough quantity to goose their April-Jun quarter numbers.)
The question about why not just discount bodies seems to get to another issue: NikonUSA may have a bit more inventory of those four lenses than they expected. The 70-200mm VR II lens sold well for a brief time prior to the Olympics, but issues with things like focal length breathing may have stalled sales (it shouldn't have, the new lens is very, very good). It also doesn't help that the exchange rate has pushed the prices of the 24-70mm and 70-200mm up. The 18-200mm sales have pretty much run their course. We're not seeing large influxes of new users who might be tempted by it--most who want or need a superzoom already have one, and the II version of that lens didn't really do anything to make us want to trade our old for the new. And the 24-120mm? Well, that lens just doesn't live up to expectations of the FX user and needs replacing.
In short, the promotions Nikon used in March worldwide all have the classic characteristics of "end of year inventory reduction." I'd go further and say that this doesn't appear to be about dollars of profit, it appears to be about holding market share (which is calculated on unit volume). This, of course, brings up the question: if sales are weak now, what is going to fix that and when? I'm betting that Nikon will have a rough first fiscal quarter in terms of DSLR market share and that the types of products that might get users primed for new purchasers won't be shipping in quantity until their second fiscal quarter (July-September).
One final thought: the fact that Coolpix promotions don't seem to be showing any signs of last minute change (or any change at all) would tend to indicate to me that Nikon thinks they'll make their Coolpix numbers just fine. The problem is mostly concentrated in DSLR bodies.