With a Nikon, I have the option of using wireless iTTL (CLS) off camera and use the camera's flash and the SB-800 in concert, or having the camera flash trigger the remote mounted SB-800 and my studio strobes that have optical triggers buit-in.
If you do this, you cannot use ETTL with Canon or iTTL on the Nikon, as the preflash that is used to test and set the light strength will set off the strobes well in advance of your shutter opening.
I do not know if the Canon can do this, but on the Nikon D200 you can manually set the onboard flash strength to anything from full strength to 1/128th power. At it's lowest setting it is strong enough to trigger my strobes but still offer no additional light to the picture from itself going off. It is also functional from a surprising range. At that low flash strength (1/128th power), it is good for an easy 40 feet area around the subject.
One of my very next purcases is a good light meter that can take incident, ambient and flash readings. The Sekonic L-358 is the one I am getting... and if in the future I decided on pocket wizards, the L-358 has a built-in trigger for them already. Too bad that I am going on vacation in 3 days... I would have taken it with me on my trip, but it has to now wait until I come back.