If you're on OS X Lion, you should be using FileVault for whole disk encryption. Enabling FileVault automatically disables Firewire/Thunderbolt DMA access while the screen is locked. You can also tweak a setting to automatically erase the FileVault key before suspending the laptop, requiring it to be entered before booting back up:

    pmset -a destroyfvkeyonstandby 1

I use

    pmset -a destroyfvkeyonstandby 1 hibernatemode 25

to always hibernate the laptop instead of suspending. The FileVault key is erased and the memory is dumped to disk encrypted.