I posted a short article about how I used Arduino to focus and take a picture with my Canon DSLR camera.
The most difficult part of this project was getting a cheap N3 connector. Canon decided to go with a proprietary plug and they charge ridiculous prices for any accessory that uses this plug. I think their cheapest device with the N3 plug is a basic remote trigger and they charge $50 for it. You could try to make your own plug. But my favorite solution was to type "canon n3 remote" into ebay and I found lots of simple remote triggers for under $10. I just got my plug from one of these devices.
With the plug in hand I started figuring out what the different wires did. Here is picture of the the plug and the color of wire attached to each pin in the plug. The white wire is ground. The green wire is +3V and when it's grounded it causes the camera to focus (similar to pushing your shutter button half way down). The red wire is +4V and when it's grounded it causes the camera to take a picture (depending on the mode the camera is in it might focus first). If you want to guarantee a quick picture you should pre-focus your camera and then set focus to manual.
Circuit diagrams, code, pictures, and more text can be found here: http://www.glacialwanderer.com/hobbyrobotics/?p=13