Interesting, I'll try to validate/reproduce this. Note however, that newer Macs are partially EFI-based (AFAIK) so this may not apply there...
Update: Oh, and this will only work for Mac OS X of course (as it's a feature in the Mac OS X kernel). If you're running Linux on Apple hardware for example (macbook, macmini, etc) then setting an OpenFirmware password will definately not help! So be careful and always test if you're affected.