Update: Since the Nikon D4 has been announced on 2012-01-06, I have also written a concise post about the Nikon D4 vs Canon 1D X.

The Nikon D3S (left) vs the Canon 1D X (right)
Canon has thrown down the gauntlet with the announcement of its brand new flagship DSLR, the Canon EOS-1D X. This new camera is an extremely important new model for Canon, make no mistake about it.
The significant stuff, in table form:
| Nikon D3S |
Canon 1DX |
|
| Max ISO | 102,400 | 204,800 |
| Megapixels | 12 | 18 |
| Max FPS | 11 | 14 |
| # AF points |
51 | 61 |
| Video |
720p, 24fps, MJPEG |
1080p, 30fps, H.264 |
| Viewfinder magnification |
0.7x | 0.76x |
| Metering sensor |
1005-pixel RGB | 100,000-pixel RGB |
| LCD display |
3″, 921k dots | 3.2″, 1040k dots |
| Price (body) | $5200 | $6800 (est) |
| Announcement date | Oct 2009 | Oct 2011 |