Why do the Nikon cameras dramatically jump megapixels?

For example the D90 has 12.3 mp and is around 899$, the D700 has 12.1 and is 2699$ and then the D3x has 24.5mp and is 7999$ I'm interested in buying a camera for around 2500 or less, I'm amateur but really looking for a camera better than decent but obviously the the best one due to budget. I'm starting to lean towards canon just because this megapixel jump is really odd. It's like you either get 12mp or 24.5mp and this seems a little strange to me.

Actually among all the manufacturers Nikon has been the most reasonable about MP figures, methinks. They didn't fall for the megapixel craze like the others did.
You can't compare the D700 to the D3x. These are different cameras for different purposes. The D700, just like its big brother, the D3S, are workhorse cameras for sports and journalism shooters. They excel in low light situations or at high iso (interchangeably). The D3X is a camera for studio use, where low-light is a non-issue. These are totally different fields of specialization of these cameras.

Frankly, since you couldn't tell this apart yourself, you'd be wasting your money with such high end gear.it'd be deprecated and outdated long before you actually need what it has to offer over the comparatively lower tiers of cameras such as the excellent D7000 (which is still quite a high end one).
Also keep in mind that DSLRs are about more than just the camera body. You'll also want additional, expensive lenses on the long run, as well as a tripod and perhaps a flash or even multiple ones. I usually recommend to spend no more than 40% of the long term budget on the camera body. You'll be much better off with a D7000 and fine lenses than with a D700 and a mediocre lens.

Don't (I repeat, DO NOT) get caught up in the megapixel numbers thing. Please.

While the megapixels will have some impact on the cost of the camera because of the technology involved at the higher numbers, it's much, much more based on the quality of the optics, the body build and the ability of the camera itself. At 12mp or more, any one of those Nikon camera will function beautifully for you. Do not waste money on a D3x unless you're going pro. Spend your money on lenses, flash, tripod, etc. All of the cameras you mentioned, again, will do you proud if you know how to use them and the XXmp won't matter.

I love the D90! I've had my hands on the D700 and drooled just a bit but didn't need to spend the extra $$. I've seen the D3 but have never touched it.

The Nikon D40 has only 6MP, and can shoot magazine quality 8" x 10"s.
If you buy a $2, 500 camera, you will be buying more camera than you know what to do with.
You'll be better off buying an entry level camera & spending the extra money on a nice lens or two.
I'd also suggest you take a photography class, or take out some books at your local library.

Since you obviously understand totally and absolutely nothing about cameras, please do not look at the nice cameras like the d300 and d7000 but yes, do buy canon. They've got a cheap plastic entry level model with loads of megapixels.

It's cause Nikon was "stuck" at 12 megapixels for many years, cause Sony was making their sensor. Canon, Pentax and others happily increased megapixels slowly, but Nikon only offered it in expensive models.

But more megapixels don't make images sharper or better. Get it out of your head before wasting money.

Here's a post called 'Buying a DSLR, what's important, megapixels, features, brand, price, quality and which one to buy - http://www.the-dslr-photographer.com/2009/03/which-dslr-to-buy.html

Good luck! The site has many more tutorials, tips, reviews and guides!