Hello Banana Jack, good question!
Nikon, like other SLR lens manufacturers so far, does not take DSLR sensor field reduction into account when quoting their actual focal lengths. If it quotes equivalent focal lengths though, then the field reduction HAS been taken into account.
So the 18mm on the 18-200mm DX is the actual focal length, and therefore is the same as the 18mm on a non DX lens. The coverage would be the same. If you were using a D200 with its 1.5x field reduction, they'd both deliver an equivalent focal length of 27mm.
The difference between DX and non-DX lenses is that the former is only corrected for the smaller area of Nikon's DSLR sensors, whereas non-DX lenses are corrected for a full 35mm-sized frame.
So DX lenses cannot be used on 35mm or full frame digital bodies - if Nikon were to ever make the latter. And non-DX lenses can work on both 35mm bodies and Nikon DSLRs, although obviously on the DSLR, you'd have the 1.5x factor to take into consideration.
The 18-200mm DX is a great lens for Nikon DSLR owners - check our review at http://www.cameralabs.com/reviews/Nikkor18200mm/
I hope you find one in stock soon!