If a normal reset pram doesn't work, the 'workaround' to reset lost passwords is below, but i'm not sure if it works 100% of the time:
1. Shut down the computer.
2. Gain access to the computer's RAM slots.
(Detailed instructions by product are available from the CIP page:
www.info.apple.com/installparts
3. Add or remove a RAM DIMM to change the total amount of installed RAM. For example:
* If the computer has two RAM DIMMs (128 MB DIMM and 64 MB DIMM) for a total of 192 MB of memory, remove the 64 MB DIMM. This changes the total memory of the computer to 128 MB RAM.
If the computer has a single RAM DIMM, you can change the total memory by adding a DIMM of any compatible size.
Note: Removing a computer's only DIMM is not an option and will prevent the computer from starting up.
4. Turn on the computer and immediately reset PRAM by holding the Command-Option-P-R key combination. Press the keys until you've heard two successive startup sounds.
5. Open Firmware password protection is now disabled. Shut down the computer and return it to its original RAM configuration.