As it turns out, it's entirely possible to hack into a VERY secured Mac, even if it has an open firmware password established. However, it would take an extremely savvy thief to know about this. Best thing to do is STILL use OFPW, and be certain to secure your Mac physically too!
If the person has PHYSICAL access to the machine, then an open-firmware password will do little to prevent them from gaining access — you can disable the open-firmware password by adding/removing a significant amount of RAM, then immediately zapping the PRAM upon the next boot. That procedure removes the open-firmware password.
The machine needs to be locked down physically as well as have software security measures in place in order to thwart potential malicious users.