The problem is not the comaptibility between Ultegra and 105 but the compatibility between the newer and older series. Your 105 shifters (5600) will work with front derailleurs from the same series (that is Ultegra 6600 and 105 5600 - probably Dura Ace 7600 too). According to Shimano, it will not work with the newer Ultegra 6700 OR 105 5700. I think the FD geometry has changed requiring a different cable pull ratio which you can only get with the new series shifters.