This is one feature the 5DMkII has that I hope Nikon will incorporate into its future DSLRs -- auto vignetting correction. Nikon introduced lateral CA correction in the D300/D700, why not vignetting too -- it is just software.
"Peripheral Illumination Correction
The Canon EOS 5D Mark II Digital SLR camera automatically conducts peripheral illumination correction when shooting JPEG images, a function that previously could only be accomplished through post-image processing using software such as Canon's Digital Photo Professional, which Canon supplies at no extra charge. Peripheral illumination correction evens brightness across the image field, making an image of a blue sky even toned throughout and reducing light fall-off at image edges. This new feature essentially eliminates one of the limitations of previous full-frame digital SLRs."
"Peripheral Illumination Correction
The Canon EOS 5D Mark II Digital SLR camera automatically conducts peripheral illumination correction when shooting JPEG images, a function that previously could only be accomplished through post-image processing using software such as Canon's Digital Photo Professional, which Canon supplies at no extra charge. Peripheral illumination correction evens brightness across the image field, making an image of a blue sky even toned throughout and reducing light fall-off at image edges. This new feature essentially eliminates one of the limitations of previous full-frame digital SLRs."