In my opinion, the D90 looks best period.
You can see the noise on the E620 even at ISO100 (while pixel peeping), which makes me wonder if it is indeed true that ISO100 in the E620 is actually ISO200 (as speculated in various forums). While there is clearly more noise in all of the E620 images as compared to the XSi, there is also more detail on the E620 than the XSi all the way through ISO1600. Here's another comparison of the E620 vs. some of its newer competitors. http://www.dcresource.com/specials/rebel_t1i-vs-d5000-vs-e620.shtml
. Unfortunately, the E620 falls behind again in noise levels. But the colors on the E620 in the images are better in the E620 than the D5000. But in my opinon, the Ti is the winner.
I am wondering if Oly should have just kept the megapixels to 10 and worked on improving noise control on the 4/3 sensor without losing the sharpness and detail that the 410/510 are famous for. But I suppose that it is hard to sell new cameras without increasing the number of megapixels, particularly to entry level buyers who are attracted by the number. Sigh.