If you have physical access to a Linux computer, then your options are unlimited. Manipulating grub could be one of it.

Things that come in my mind :

- Use USB Flash/CD to boot something else, mount the hard-drive and do some nasty stuff to passwd file.

- If you have your BIOS locked, you can simply remove the hard drive and use another box to do the same.

- If you got your BIOS locked with a password, you can simply remove the battery for 1-2 minutes and put it back!

- No access to the battery?! Remove the hard drive, boot from CD/usb key ( since there is no hard drive almost every BIOS will search for something to boot from ) and flash the BIOS :D

No system is secured if you have physical access to the hardware.