I was just recently trying to explain this to a European friend who thought I was hallucinating this system, so I took a picture.
http://dl.dropbox.com/u/230717/temp/208YPanel.jpg
That's a picture of one of the breaker boxes in our office, showing what you described. There are 3 phases coming into the panel, each a different coil off a Y transformer, as well as a "neutral". Those are the 4 black wires you see at the bottom. You can see how the three hot phases are staggered as they go up the breaker rails.
For standard 110V service, you use a single-wide breaker and send one hot phase + neutral and you get 110V. The difference between two phases is 208 volts though, so you use a double wide breaker and can send to device without using a neutral wire. Just 2 hots and a ground. If that's all you're doing (you don't need legacy 110V service anywhere) you skip the ground wire going into the panel entirely.
that one looks dangerous. In europe: http://img406.imageshack.us/i/verteilerkasten.jpg/ 64A 240V 3-Phase input. Out to Servers single phase, output to airconditioners with 3 phase (not at this picture). Kind regards, Ingo Flaschberger