As in the subject line -- does Facebook really enforce that your profile picture be a photo of yourself? If so, where have they historically drawn the line with respect to gray areas (such as a photo with only your shoe, or a cartoon-ized or photoshop'd image of yourself)?
From seeing the swathe of cartoons, bikes, other people, group shots, landscapes and whatever else, it looks like pretty much anything goes.
Just so long as you abide by the Facebook Terms of Service and that you have the right to distribute the photo (i.e. you own the photo) you're fine to use whatever photo or image you want. Though even the latter stipulation isn't really regulated.
If it's the kind of image that would be on the line for obscenity, nudity or vulgarity, then it would be a problem if other users flagged the profile photo.
No, it doesn't look like they enforce the rule that it must be your actual face on the profile.