For the very purpose of storing your opt-out preference, they would have to keep a record of your facial features, or you would continuously come up again as people tagged you manually.
Actually they already allow you to opt out of the facial recognition feature:
"Who sees tag suggestions when photos that look like you are uploaded?" ("Friends" or "No one")
You can also set Facebook to require manual review of friends' tags of you. I'm not sure if there's a way to completely opt out of being tagged though.
From an article I found,
* Shadow Profiles: Facebook is collecting data about people without their knowledge, using it to substitute existing profiles and to create profiles of non-users.