You know, I don't think there is one on an iBook/PowerBook keyboard. You may be able to map it to one of the function keys though in the System Preferences>>Keyboards and Mouse panel.
Strickly speaking, no, but I use several apps on Mac that do support overwrite. Microsoft Word for example, but you do it by clicking the "OVR" box at the bottom of the page. Also emacs, which I use alot, supports it, but I have to get toggle it from the command line rather than by a key press.
On the regular Apple Keyboard, the "help" key is the "insert" key. However it only sometimes acts that way. If you notice on new PC keyboards, the insert key have been moved.