D600 is using Full Frame sensor (FX). 24mm will give you actual 24mm.
D90 is Crop Sensor (DX). The crop factor is 1.5X, so whatever lens you mount, you need to multiply by 1.5X to get the actual focal length (in 25mm format). So your 18mm, when mount on your D90, the focal length will be 27mm.

Other than that, 18-105mm kit lens on the D90 is a DX lens, while the 24-85mm kit lens that comes with D600 is a FX lens.

Ok lor, mine's old model. You also own Peugeot?

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Added on November 28, 2012, 9:06 am

Hehe, those can compare by specs. I think the main confusing thing is DX lens vs FX lens. Some people may confused that DX lens means no need to multiply by crop factor when mount on DX body, but actually you still need.
But when you mount DX lens on FX body, then it will be using CROP mode, which means you need to multiply by crop factor. So a DX lens at 18mm, will give you 27mm regardless you mount on DX body or FX body.