Congrats on your choice. I have had a d80 after I upgraded from a d40 several months ago, and have been very happy about the upgrade. Here are a few things you will notice about the d80 vs the d90.
1) The metering on the d90 is better than the d80. One thing I noticed is that the matrix metering on the d80 is a little inconsistent. This is mostly apparent if you are shooting some drastic contrasts in the same shot. The other thing that sometimes gets annoying is that the shots taken in the same conditions will sometimes have a different metering in matrix mode. The d90 has a much better metering system in matrix mode. Personally I fix this by simply shooting mostly in spot metering or center weighted mode. I shoot mostly portraiture so this is fine for me, although people who shoot a lot of landscape images tend to be annoyed with the inability to apply the same post development to all pictures taken in the same conditions.
2) The ISO on the d90 is better than the d80. I don't shoot in really high ISOs personally, but there are times where a few extra ISO points would help me out especially in tricky lighting conditions. There is an indoor swimming pool that my son takes lessons at that I shoot pictures in that I could use the higher ISOs, but for the most part it isn't a HUGE difference unless you are shooting professionally.
These were the two major drawbacks I had, but at the time the D90 came out it was just at 1,000 for body only. My d80 was just at 600 bucks because it had just dropped in price because of the d90 release. Again it all depends on your shooting, but I think most entry level DSLR users could do much worse than a d80. 🙂