I would not recommend it unless you have a lot or are planning on purchasing a lot of "D" lenses. Even if that is the case, I think you should consider the D90 or D7000 over the D80. The D3100 is a fine camera and is much lighter than other bodies. The D80 is a great model with a bright viewfinder and an in-body motor, but it actually gives up some IQ to the D3100 and can't do video. This is because it is a different generation body.
It might help to understand the Nikon DX dSLR lineup:
1. D40x/D60 -> D3000 -> D3100 : Entry class DX dSLR
2. (D60?) -> D5000 -> D5100 : Recently characterized by the flip down LCD for live view and videography
3. D80 -> D90 -> D7000 : Enthusiast model. Adds an in-camera motor to AF older lenses and a bright viewfinder
4. D200 -> D300(s) -> (nothing) : Professional DX dSLR. Adds an even more expensive viewfinder, all-metal construction, and better performance. Metering still works on older lenses.
My personal history might help. I started with the D70 (two generational predecessor to the D80), jumped to the D200, and then left for the Nikon FX line (Nikon D3). To be honest, the D200 is more than enough camera for me today, and the image quality out of your D3100 would blow my D200 out of the water!
Why not give it a little more time. As you graduate up in Nikon bodies, you gain more physical control over the camera and better performance (more fps and brighter viewfinder), but you lose a lot of the automation features (guided mode, scene modes, etc). Besides, any lenses purchased for the D3100 will still be excellent on a future DX body. (I still shoot my DX lenses on my old D70 which I use for infrared photography).
If you find the menu-based features of the D3100 confining, never use the guide mode, and want more direct control, you can purchase a D5100, D7100 or sequel to the D300 and sell your D3100 once you get used to the extra weight and know you aren't giving anything up in performance.
I hope this helps and good luck with your Nikon camera, no matter what model it is!
Jan 9, 2012 by
San Francisco, CA
San Francisco, CA
Favorite Subject: Landscape
Nikon Family: 6-10 years
Experience: Less than a month
Role: Semi-professional photographer