Congratulations. You have come to the kings attention as one of the top land owners in the realm. He has requested that you become his direct subject. Will you join the king?
By my recollection I started getting this flattering offer some time around Marquisa. It sounds like it should be a great deal, you know -- cutting your money grubbing leige out of the loop -- but before taking it I asked around.
Unless I'm being misinformed, people who serve the king still pay taxes every month at the same rate (prince=1%, duke=2% etc.) as they did when under their former leige. It just all goes to the king. And he still gouges you every December, just like normal.
Someone told me that you get more slots to stick some subjects in, but that's not really important to me as I haven't filled all 10 and don't plan on it.