Nikon D5000 vs Nikon D90: JPEG Image Quality – Reading through Thom Hogan's article titled "D3000 and D300s Leaked–Predictions Updated", you'll find this paragraph:
What looks a bit weak in the expected lineup is the D90. The D5000 is so close to the D90 that the D90 looks a bit strange in this lineup, especially when you consider that the D5000 seems to produce better JPEG image quality than the D90 out of the same sensor.