Like Mike I am also a Canon man. I am surprised the Nikon folk have not rushed forward to sing the praises of the D90. Whilst I don't believe in getting the latest gadget for its own sake Alex does verge on the technophobic.
If you have the cash burning a hole in your pocket then sure go ahead the photographic industry needs the cash injection. If not then you need to take a step back and think it through. There are the folk who buy the latest model every 18 - 24 months or so and it is not long before the shine wears off and they are lusting for the next model. Is your camera a tool or a piece of jewellery ?
When buying a new piece of kit I am a strong believer in understanding exactly what advantage it has over the old kit. It maybe a better quality lens will go along way to improve your photos than a new body.
What features of the D90 do you need ?
Sure the D90 is a step in the right direction above the entry level D60 but will it improve your images or do you want a more robust body with a few more and easier to use features.
From your question I suspect you are not sure.