Remap the 'UpperCase' key
In general, remapping a modifier key to another modifier key is relatively simple; Apple provides a system preference pane to do it. Remapping a standard key to another standard key is slightly more complicated, but easy enough by editing or creating a keyboard layout. The tricky part is converting a modifier key to a standard one, or vice versa. For that, you need a specialized program.

You will need a System Preference Pane called PCKeyboard Hack.

First, go the the Keyboard preference pane, select the Keyboard tab, and click the Modifier Keys... button. In the resulting sheet, set Caps Lock Key to No Action.



Then, install PCKeyboard Hack.

In the preference pane it installs, go to Caps Lock, select "Change Caps Lock", and set the key code to 48 (the key code for the tab key).

Instructions on a related question are available on Stack Overflow.