The D90's successor has appeared: the D7000, poised to challenge Canon's 60D. It's not strictly a successor because the D90 is still available but I bet the D90 will be phased out soon.
The D7000's specs seem quite advanced and outdoes the D300 and D300S in a few areas such as resolution (16MP, 1080p video with full time autofocus, and having ISO 100 as the base ISO). No articulating screen like the 60D but it does have 39-point autofocus and dual SD cards. I'm still reading about it.
I'm still waiting for a replacement to the D700 (hopefully with a D3S sensor), but so far, nikonrumors.com disclaims any such camera this year.
UPDATE: I've read a bit more about the D7000 and it's so well-spec'd that now I'm thinking that it may even replace the D300S. The D300S doesn't have many advantages over the D7000 - in fact I can only think of the 51 vs. 39 AF points. The D7000 even has 1/250 (1/320) sync speed, a 100% viewfinder and AF fine tune just like the D300 and D300S! On the other hand, the D7000 has several advantages over the D300 such as the higher resolution (16mp vs. 12mp), the more advanced metering (2016-pixel RGB sensor vs. 1005-pixel RGB sensor), and the lower price.
If the D7000 does replace both the D90 and the D300, I think that would make sense because it would create a bigger gap between the prosumer D7000 and the pro-level D700. This is a great time to be in the market for a mid-level camera is all I can say....