Hi
Instead of asking D90 or 50D, why not also ask which lense system do you want? If you are willing to part $1800 for a body, I bet you will be getting some really solid lenses! Hay this is 50D, not 1000D!!!
So, most importantly, ask yourself, how much are you going to invest in the next 3 years in photography and plan your lens and decide which system to get.
I was asking myself, for 50D, I can get the legendary 17-40L which can be used as a fullframe (if I am not wrong). Cool! And of course, the 17-40L is very very good in terms of optics! I looked at the other lens of canon of same price range and well, lets just say 17-40L at that price is a rare find.
I finally decided on D90 with 16-85VRII. Of course, this combination save me quite a few hundred of dollars (or issit close to 1k?). ( FYI, I am truly enjoying the 16mm wide.. 2mm at 18mm is substantial vs 2mm at 85mm

Its the angle that counts!)
To be frank, getting a 50D with the $100-$200 canon kit lens is quite a mismatch ( I will knock any salesman's head if he will to intro me this).
You are not buying a compact, especially if you are getting 50D. It is 30% the body, 70% the lens. If you take RAW, it is 90% the lenses and flash, 10% the body! If you really want to compare, I think the ranking is as such
D300 --> N
50D --> C
D90 --> N
40D --> C
D80 --> N
450D --> C
D60 / D40/ 1000D market
So it is pretty obvious the market strategy. The cameras are not equivalent or comparable!
Side Note: D90 LCD screen is awesome for viewing Pic U took!