FileVault 2 is supposed to be secure against this* when the machine is powered on and locked or sleeping.
* Source http://security.stackexchange.com/questions/18720/how-secure...
* Source http://security.stackexchange.com/questions/18720/how-secure...
> If you enable LUKS root, DMA attack mitigation is also enabled(boot.initrd.luks.mitigateDMAAttacks ). It consists of blacklisting firewire drivers.
Edit: still at risk of the "Evil Maid Attack" http://www.aspecrypt.com/evil_maid_attack.html