Root user able to access FileVault 2 encrypted drive
Once you've logged in using one of the users set up for FV2, the unencrypted data is available to all users on the computer. All of them: root, admins, regular users, and non-login users like _www or _spotlight.

If you make the disk available to another computer, either by physically moving it or using target disk mode, then no one on the other computer, not even root, can access the data.