Just ran across this today that Facebook will soon be doing away with the "Become a Fan" button and instead changing it to a "Like" button. So, essentially, you will still become a fan of the page, but instead of saying you are a fan of a brand or business, you are indicating a Like for this brand or business. It will still show up in your Newsfeed and on your profile, they're just changing the wording.
This means that for any client fan pages where you may have designed a custom tab which included the call to action of "Click on the Become a Fan" button above, it will need to be revised to reflect the newest changes once Facebook implements it.
What is not clear, is whether people who "Like" a page will still be considered Fans or if they'll be considered something else.
You can read more about this upcoming change
HERE.