Do All Nikon DSLR's Use the F-Mount?

I'm considering buying a Nikon dslr and was wondering if I buy lets say a Nikon D7000 and I purchase many lenses for that camera will I be able to use them on a Nikon D3X? I know that the Canon line of DSLR's does this with their ef lens mount and was wondering about Nikon.

Yes, all nikon DSLR's use the F-mount. But, there are nikon F-mount lenses, designated "DX" that are designed to be used on nikon's DX cameras, like the D7000, like the AF-S 35mm f/1.8 DX. However, non-DX lenses, like the AF-S 50mm f/1.4, with an image circle large enough to be used on a full frame, or FX camera, can also be used on a DX camera. (it's a little confusing, I know) DX lenses produce smaller image circles to match the smaller sensor of DX cameras in an attempt to make them smaller and lighter. Since the image circle is not large enough to cover a full frame, it can't be used on a full frame camera unless that camera has the auto-mask function, which the D3x does. This function masks the area not covered by a DX lens and automatically crops the frame, producing a lower resolution image, at I believe 11mp on the D3x, which is normally 24mp. Canon's equivalent of the EF mount for crop frame cameras (like the rebel and xxD series) is designated EF-S, and works the same as nikon's system, EF lenses can be used on crop cameras, EF-S lenses can be used on crop cameras, but can't be used on canon's full frame cameras (5D and 1D series) due to vignetting.

If you know that you will eventually move up to a full frame camera, my recommendation is stick to non-DX lenses.

Yes, they do. Nikon has been using the F-mount since the Nikon F was introduced in 1959.

You will probably want to make sure you purchase FX lenses instead of DX. While DX lenses will work on full-frame DSLR's like the D3X, you may find that there's vignetting in your photos. That is because the DX lenses are made for DX bodies, which have smaller ('cropped') sensors than the FX bodies, which have sensors that are the size of a frame of 35mm film.

I should also point out that Nikon's FX DSLR's do have the capability of shooting in DX format to eliminate the vignette effect. Personally, I'd much rather use FX lenses on a full-frame body like the D3X.