What I do;
I have all my movie files on an external hard drive. I then add these to itunes without importing the files onto the macbook. Leaving the external hard drive attached I then copy the files manually across to ATV... (I think you need to turn off automatic syncing).
Disconnect the hard drive... You can now watch all the movies etc. which are on the ATV without having to have the macbook on.
Only bad thing is you're dependent on the ATV hard drive size (unless hacked). But means you don't have bulky files filling up your macbook. As far as I know you can't get files back off the ATV which is why you should keep them on the external hard drive.
Any better solutions please let me know...