Canon did a risky thing at the dawn of autofocus by starting fresh and eliminating backward compatibility for their older lenses. Nikon, for contrast, allows all the ancient lenses to more-or-less work on modern hardware. This has created an alphabet soup of different Nikon lens standards that you need to at least vaguely understand if you want to buy used gear. Up until recently, Nikon went to great effort to make sure that new lenses worked well on ancient bodies. With the newest G-series lenses, this is no longer true. The Nikon G-series lenses no longer have an f-stop ring on them, so they require a newer body that can do all the adjustments electronically.

On the flip side, you apparent can still plug a very old manual focus lens into your modern Nikon D70. The main thing you're giving up is that you won't have any of the "CPU" functions. Or, in other words, the lens can't tell the camera "your subject is 10 feet away", which the flash system uses to adjust flash brightness. Of course, if you're using some ancient lens, you can't exactly expect to have all of the modern features.

If you're using a new body and a new lens, then you're pretty much in the same world as Canon people. One intriguing difference between Nikon and Canon is that Nikon has decided that they're apparently standardizing on the 1.5x multiplier for using smaller sensors. Canon seems to use a slightly different size sensor in each of their cameras. The benefit of Nikon forcing a standard size is that they can engineer new lenses around it. The Nikon 10.5mm fisheye, for example, is far smaller than a full-frame 10.5mm fisheye lens would otherwise be. This gives Nikon all the same benefits that Olympus talks about for their new 4:3 system lenses.

Canon, for contrast, only makes one lens tuned to smaller image formats, and that's the 18-55mm kit lens for the D-Rebel. Sigma just announced an intriguing 18-125mm lens that's also tuned for smaller image formats. If you're the sort of person who would have bought yourself a 28-200 lens for a 35mm camera, then Sigma has an answer for your Canon, Nikon, or other smaller-format D-SLR.

At the end of the day, you need to draw a picture of your platonic ideal camera. What features do you require today? What sort of expandability do you need in the future? Are you more interested in light weight or in heavy duty? How important is it for you to be able to rent exotic lenses? To purchase or sell gear on the used market? What's your budget? Etc., etc.