If you want this functionality in a page tab or canvas page within facebook (say to allow for liking the page from within a likegated page), a work around you can involves what Tom Wells suggested in his reply to Luke. You first embed the iframe version of their like button on your page, and then simply listen for the edge.create event in your JS like so:
FB.Event.subscribe('edge.create',
function(response) {
alert('You liked the URL: ' + response);
// ...
}
);
In the callback, you can deal with with what happens when the user has liked the page, say like navigating away from the like-gate page, or showing liked-only content.
When the user clicks the iFrame like button, your JS code should receive the edge.create event assuming the iFrame was configured to point to the url of the page in question.