With FileVault enabled, do I need to overwrite my hard drive with 0's before getting rid of it? When I give up my laptop I like to securely delete the hard drive. I would assume that since it's full disk encrypted using file vault that I should be able to just delete the real key for boot partition and be done. No need to write 0s over the filesystem.

Is this true, or do I need to do something extra to make sure that I can safely part with my hard drive?
No - the point of FileVault 2 is to make it extremely difficult for a motivated person to decipher what was stored on that volume.

I would pay more attention to destroying that key that was used encrypt the data than the drive.

Of course, you could zero the drive, but why not go one better and pick a new encryption key that was never used, and never will be used and let your machine re-do the encryption of the drive once you have erased all the data one pass.

Since there have been no scholarly articles on how easy it is to decrypt a FileVault drive nor reports of exploits - you have to weigh the potential damage that leak of information could be. FileVault makes it much easier to not worry if you lose a drive, so the old saw to be sure to overwrite things many times is far less applicable for drives where encryption exists at rest.