There are no strong rumors, except that the D70-80-90 upgrade schedule is every 2 years, and it has been about 2 years since the D90 came out.
Possible release dates are anywhere from may to December though. To be honest, I wouldn't wait too long to find out if a replacement comes, there is always a new camera around the corner.
The D90 replacement will be more expensive than the current d90 list prices and the D90 wont drop in value for new prices when the replacement is released.