The new Nikon D5100 was just announced today, replacing the D5000.  It sits just above the D3100 in the Nikon consumer dSLR lineup, and a bit below the D90 and D7000, both sturdier and more sophisticated cameras.  The D5100 boasts a 16 megapixel sensor (like the D7000), and a swivel screen more like the Canon T3i and 60D, attached at the left side, rather than the less convenient bottom swivel of the previous D5000.  (see my post comparing several of the latest Canon dSLR cameras here, and one comparing the Nikons here.)

Nikon D5100 dSLR

The D5100 shoots 4 frames per second, faster than the 3 fps of the D3100, and of course has 1080p HD video.  The D5100 now has in-camera processing filters like those of the 60D and T3i (including miniature, high or low key, and selective color in the Nikon).  And it has the 11 point AF system, with one cross type sensor, like the D3100.

To compare the Nikon D5100 vs. the Canon Rebel T3i:
(click the camera names to view them on Amazon):

Canon Rebel T3i / EOS 600D
18 megapixels
Articulating rear LCD screen
1080p HD video with all the frame rate options
9 point autofocus system with 1 cross-type sensor
3.7 frames per second continuous shooting
In-camera image processing filters plus RAW to JPEG, cropping, and other options
Some scene modes
ISO range 100-6400
95% viewfinder with .85 magnification
1.26 lbs with battery
Wireless flash triggering

Nikon D5100
16 megapixels
Articulating rear LCD screen
1080p HD video with all the frame rate options
11 point autofocus system with 1 cross-type sensor
4 frames per second continuous shooting
In-camera image processing filters
A lot of scene modes
ISO range 100-6400
95% viewfinder with .78 magnification
1.2 lbs with battery
Also an auto HDR feature where the camera combines 2 images to create an image with extended tonal range.

As you can see, both cameras are incredibly similar as far as specifications.  It looks as if the price may even be the same.  A deciding factor may be which camera feels more comfortable in your hands, which controls and menus seem better placed and easier to navigate, and which system you want to invest in.  A look at the image quality here seems to show that the Canon’s JPEG images straight out of the camera are sharper, have more clarity, and more color “punch.”

If you decide on either of these cameras, please be sure to check out my e-books Canon T3i Experience and Nikon D5100 Experience, and learn to use your dSLR quickly and competently!