According to the TOS (http://www.facebook.com/brandpermissions/logos.php http://www.facebook.com/brandpermissions/logos.php) you can change the size but nothing

According to the TOS (http://www.facebook.com/brandpermissions/logos.php http://www.facebook.com/brandpermissions/logos.php) you can change the size but nothing else. "While you may scale the size to suit your needs, you may not modify the Like Button in any other way (such as by changing the design)."