Going off of a tip to disable V-sync for better FPS, it seems that the mouse tracking goes to the default speed (which is waaaaay to slow for LoL).
For reference, I'm running an iMac 20inch 2.4 2400XT 4GB on 10.8.2.
Disabling V-sync gives you the ability to adjust your mouse speed.
Open menu, more options, and check "use software mouse".
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.