I did a cavalry topic a while ago, I still need to update it at some point to deal with tips on attacking enemy soldiers from the front, I've managed to find a tactic which works about 25-50% of the time when attacking infantry head on (the percentage depends on the experience of the enemy soldier), while its not perfect its the best I can do in terms of frontal cavalry attacks especially with light cavalry.
Gameplay mechanics have been changed with the patches since I posted this: http://forums.taleworlds.com/index.php/topic,227456.msg5445615.html#msg5445615