Unlike most of the yearly ho-hum updates out there (a few megapixels here, a couple of features there), the EOS 5D Mark II announced yesterday is worth the three-year wait: the full-frame sensor has been boosted up to 21.1 megapixels, the LCD is a high-resolution 3-inch beauty (like more recently released competitors), Live View shooting has been added, the sensitivity range has been raised to ISO 50 - ISO 25,600, auto focus points have been bumped from 9 to 15 points, and continuous shooting speed has been bumped from 3- to 3.9fps. Best of all, however, Canon has added HD (1920x1080,16:9) video capture at 30 fps (there's also a standard TV quality 640x480, 4:3 setting as well), besting recently announced competitors like the Nikon D700 and Sony Alpha DSLR-A900. In fact, the consumer-oriented Nikon D90 is the only other dSLR to offer a video mode at the moment.
Canon plans to ship the 5D Mark II by the end of November for $2,699 (body-only) or in a kit version with the EF 24-105mm f/4L IS USM zoom lens $3,499, making it very competitively priced compared to the competition.
For an excellent features comparison among the EOS 5D Mark II, close competitors, its predecessor, and even its big brother the EOS 1Ds Mark III, check out Lori Grunin's post at CNET crave.
For more details than you ever wanted to know about the camera, see DPReview's comprehensive hands-on preview.