Facebook introduced a change in the Fan Page today. They have replaced the "Become A Fan" button with a "Like" button. Earlier fans of a page are now shown as having liked the page.
Earlier people "Become A Fan" of Facebook Pages. With today's change, people now "Like" a page. However, this "Like" is different from liking a status update, post etc from a friend. You will recieve updates from the pages that you have liked as was the case when you become a fan earlier.
This is what Facebook has to say about the change they have introduced:
You can read more about it here.