Happened last night. I have an AMD M880G, with an ATI Mobility Radeon HD 4200 card, and was playing as Karma. I don't remember when it happened, or the circumstances, but I tried using low cursor tail as suggested by a hastily googled forum poster (mid-game), and it brought the cursor back in windows, but when I opened LoL back up the mouse was still gone. Had to restart the computer to get the mouse back post game. Luckily, I didn't get reported by my team for AFKing for a short time, or failing to buy late game items because of the lack of cursor, or failing in late game teamfights without said cursor (very hard to hit exhaust or your shield), and the team was sympathetic to the problem. This might not always happen though.
*Needless to say, we were winning, I was 2/0/8, then we lost*