Leon Powe has my vote for MIP.
i love powe, but his leap in ability is second to rondo's. not only did rondo improve in every area by leaps and bounds, but he did it as a starter with the weight of a franchise on his shoulders. We all take him for granted now, but what if he HADEN'T improved himself the way he has? what if he was making typical 2nd year PG mistakes, turning the ball over, and still couldn't hit a jumper? Imagine where we would be, it wouldn't be out of the playoffs, but it certainly wouldn't be 1# seed with a chance to win it.
Leon had a fantastic year. Rondo had a fantastic year under an incredible microscope, and was told he would fail by anyone who would listen. Rondo wins my vote for most improved player, hands down.