Is it possible to have a Mac drive a display as a mirror image (and not simply rotated)
Do you need to flip an built-in display (like on an iMac or MacBook), or an external display?

I'm afraid it's only possible to flip an external display. Here's how to do that (tested with 10.6, but 10.4 should be similar):
Go to System Preferences-> Displays. Go to the Display tab of your external monitor's window. For Rotation: select either 90 or 270.

You can still mirror your displays.