as i learn more i realise that changing your body is probably the biggest step you'll take with regards to your kit. personally i feel that if i do ever get the hang of my d90, then i'm in a position to move on, but to be honest, why get rid when you now have a tool that you can confidently use and concentrate on more important things like composition etc? i do the www window shopping and have filled many a basket with a d700, but common sense takes over and the i'm fine for a week or so!
i think there's many people who might regret buying a new body when they suddenly find out they are back to square one ?
Anyway, thats just my take on it.