Thom Hogan has released his 2010 predictions (see source link below), and is predicting a D90 Redux, although he expects it to be called the D8000 instead of D90s.
He expects a different sensor, different AF, and improved video.
Our take: Yes the D90 hits 2 years old in 2010, so a redux is due. If Nikon introduce a new sensor we'd expect it to be the D7000 not D8000 (since Nikon likes to bring in the "upper end" models starting with 7 - i.e. the advance consumer bodies went D70->D70s->D80->D90, and the pro-sumer FX bodies started at D700).
However Nikon has been heavily reusing sensors to help control costs (think D200->D80->D40x->D60->D3000 all with derivatives of the same sensor, and so far the D300 sensor has been seen in the D300->D90->D5000->D300s). Given the D90's massive success, we still believe it more likely to see an iterated D90s rather than a new body/sensor (either D7000 or D8000).
Source: Thom Hogan on 12/4/2009.