The Canon EOS-1Ds line has been ruling the roost of high-resolution studio digital SLRs since it was first released six years ago, amidst full-frame competition from the likes of Kodak and Contax. Both of those companies have since left the digital SLR market, leaving the EOS-1Ds as the only full-frame 35mm camera until Sony's A900 last year and Nikon's D3 earlier this year. With their studio/landscape model, the D3X, they have finally won back the resolution crown from Canon at the very-high-end.
The 24.5MP full-frame D3X lists at the same release price of the EOS-1Ds Mark III: $8,000. Having already been on the market for over a year, the EOS-1Ds has dropped in price to around $6,500 meaning there's a de facto $1,500 premium on the D3X at the moment (this too will decrease over time). Other than the resolution—and obviously lens mount—the differences between the D3X and EOS-1Ds Mark III are very small. The D3X has a larger buffer and maintains the "hi-speed" option introduced with the D2X to capture at a higher frame rate (7fps) with a reduced resolution (mimicking the DX-crop).