Just took the Nikonians i-TTL flash workshop recently and wanted to give it a plug (I don't work for them). This was a GREAT class and I learned A LOT of valuable info on the Nikon flash system. This workshop covers the use of the SB600 /SB800 and Nikon Creative Lighting system. I'm an intermediate level photographer and walked away with a lot of valuable info (will be buying a second SB800 soon!). The workshop covers everything about i-TTL but what I found most useful was the CLS and multiple wireless flash photography, particulary in the context of a highly portable lighting studio (2 SB800s, umbrelles and stands = professional lighting studio in a backpack). The cost of the workshop is approx. $95 and workshops are being taught in N. America and Europe. It's geared toward D70 users but anyone with an i-TTL capable Nikon will benefit from this.
More info:
http://www.nikonians.org/html/workshops/D70/d70_content.html
More info:
http://www.nikonians.org/html/workshops/D70/d70_content.html