Well the D700 will be replaced first. The proof is that supply is already drying up. I'd believe you mb but the only problem with your "plan" besides what foofie already pointed out (D5000 is not even a year old) is that the D5000 hasn't been heavily discounted. The D90, D700 and D300S have. The D5000 is Nikon's number 1 selling DSLR in some countries followed by the D3000. (In some places it's the other way around)
It would be a huge mistake for Nikon to keep the D90 around until Christmas! Despite that it's barely going to be 2 years old, it's already pretty outdated. Remember it was the world's first HD video DSLR. At the time nobody else had HD video in a DSLR. Now look at how many other cameras do it "bigger and better". Nikon can't afford to wait 6 to 8 months for a replacement.
Umm not everyone buying a D90 is a amateur consumer... Canons consumer t2i just barely equals the specs of Nikons 2 year old model (compare the specs, and Mpx is only big on the consumer end).
While I agree that a new model is definitely needed before Christmas, video doesn't seem to be the main reason buyers purchase this camera as it is only offered (at least where I live (LA county as a whole)) in camera stores or stores that carry the D300s too. The D90 is "pro"sumer, the main buyers seem to be intermediate and advanced amateurs, Advanced students, and low budget professionals, not walmart camera shoppers.
The D700 is a mini D3, the D300s is a smaller D700, the D90 is a compacted D300s, and the D5000 is a simplified D90, but the D5000 is sure not anything like the D3! The D90 is the cut off line where consumer modes top out and professional settings begin...