From Chaotic Software:
The Startup Security application enables firmware security on your Macintosh. Available for Mac OS 9 and X, you can protect your computer from harmful use by changing a few simple settings. When Startup Security is combined with a physical security solution, your machine virtually becomes an impenetrable fortress!
Set a firmware password so that boot device settings and other machine parameters cannot be changed without authorization
Completely prevent boot-up of a machine without the proper password
Enable three different modes of security -- Off, Default Boot Device Only and Full
Combined with recent Macintosh firmware updates, you can use Startup Security to provide a greater level of security for your system.