The introduction of d7000 is kinda of confusing ... it is not suppose to replace d90, and it stood slightly in between d300s and d90.
I think d7000 was introduced to TOP the 'thousand' models like d3100, d5100.
In the past before the days of D5000, D7000, and 7D, there was sort of a misalignment in terms of competition between Nikon and Canon.
Canon's 40/50D feature set sits in between Nikon's D300 and D90.
Canon's 500/550D feature set sits in between Nikon's D90 and D60
Canon's 1000D sits very slightly below Nikon's D60.
This is evident in the features offered, as well as the price they are offered at. But then, as we all know, people are kind of funny. They like to compare, and make direct 1 to 1 comparisons. This makes some models look bad or good depending on how it was worded. Eg. D90 is so much more ex than the 550D! D90 is not as good as a 50D! 50D cannot touch the D300 in performance! etc etc and the list goes on.
So Nikon released the D5000 to address the upper entry level gap to go head to head with the Canon 550D. The D60 is repackaged and launched as D3000 to go head to head with 1000D. Canon on the other hand, launched the 7D to go head to head with the D300s. I believe that Nikon's launch of D7000 is to go head on against the 50D. However, it is just surprising that Canon decided to dumb down the 60D by removing features like mag alloy body and AF fine-tune, I think probably in fear that it will cannibalize the market for the 7D. This made the D7000 so much more attractive in specs and features than the 60D now. However, there is definitely some cannibalization of D300s' market. I believe that Nikon is not too concerned about that, since the D300 series product lines are nearing the end of their life cycle.
So in the end, it is just a re-alignment of the product mix for both companies.