I'm another guy who went from a D70 to a D90. It is a great camera. I agree with Henrik overall, and am in the same boat as Gary. The only thing causing me to step away from the D90 is the AF of the D300s, or possibly a D7000.
I think the D90 is a big jump up from a D70, in pretty much every way. Generally glass is always a better and more enduring investment, but the D90 gave me a lot of shooting flexibility I just didn't have with my D70.
Net: you won't regret the upgrade, but don't get into the habit of chasing bodies. Invest in glass. It will work on your next 3 cameras.