Recommend the next 'step up' from the Nikon D3100?

I don't exactly have the money to go out and buy a new body yet, so this is more curiosity than anything else!

I would say the D7000, or the D300s or if you want full frame, the D700 or D800.

You have a camera that should keep you busy learning the fundamentals of photography. You need to be able to use your existing camera intuitively before spending any more money on a new camera.

Most people buy additional lenses before buying another camera.

If your goal is to take the next step, the Nikon D7000 would be moving from an entry level camera to a semi-pro camera like the D7000

I would say the D7000 for sure, or the D90, depending on how professional you want to go, or what your budget is.

The D7000 has 16.2 megapixels, 2x SD/SDHC, 3" LCD screen, Full HD 1080p video, 6 frames/s continuous shot.

The D90 has 12.3-megapixel DX-format CMOS imaging sensor, continuous shooting as fast as 4.5 frames-per-second, low noise ISO sensitivity from 200 to 3200 and 720p HD video.