I agree and disagree.I feel certain things should be limited to members, a like feature being one of them. How do you track them? If they've already liked a post? It becomes hard and inefficient. I vote for no, and will disable it if its an option.
cookie-based system (HTML5's local storage anyone?) will stop most people from liking posts multiple times as they have no idea how to delete cookies. We just omit guest 'likes' from trophies etc.I'd vote for NO.
How can you be sure the same visitor won't like that post more than one time? or even the user like his post as a visitor. Definitely NO.