Is there a way of "re-locking" an internal FileVault 2 encrypted partition? As an alternative to using encrypted disk images, I created a FileVault 2 encrypted partition in Lion for some important data on the SSD in my MacBook Air. The password is not stored in a keychain. Once it is unlocked, I can unmount and mount it again without authenticating, even after logging out and back in again. Nothing short of restarting seems to lock this partition again. 

Re-locking an internal encrypted partition while logged in is possible in other implementations of whole-disk encryption, and for FileVault 2, according to the man page for diskutil, specifically, for the command diskutil coreStorage unlockVolume,


  "This verb unlocks an LVF by providing that authentication; as the LVs are thus appear
  as dev nodes, any file systems upon them are automatically mounted. To "re-lock" the
  volume, make it offline again by ejecting it, e.g. with diskutil eject"


While this does work as it should with encrypted external drives, it does not seem to work on my encrypted internal partition. I can eject the logical volume, which is 
/dev/disk1, but it does not get locked. Trying to eject its containing Logical Volume Family, Physical Volume, or Logical Volume Group, results in errors, even using administrator privileges. I tried variations, using unmount, unmountDisk, referring to the device UUIDs, with no luck. What am I doing wrong here?
Due to the limitations, you might want to use an encrypted disk image rather than whole disk encryption.

Come to think of it - why not use both? 

You could always turn off FileVault if you felt it too much a hassle, but to get around the limitation you caught (thank goodness you did catch it),  consider using Disk Utility to make secure the parts of your data you need to force into a locked state when that disk image is unmounted. This will let you sleep your mac and not compromise that data by allowing it to be re-mounted without the appropriate passcode being supplied.