Overcharging any battery shortens it's lifespan. Lithium batteries are the most intolerant of overcharging. The overcharging creates heat, which helps destroy the proper chemical function of the electrolyte in the battery. Enough overcharging and you can catch one on fire, but I believe that won't happen with the current charging system.
Overheating actually causes a chemical change so the the battery cannot store a charge as well... also it usually also drys out the battery electrolyte in most cases.
The problem is that the charging circuit takes a while to determine how much to charge the battery, and so it charges "blind" for a while. If the battery is already fully charged, then it will become overcharged.
Hope that is enough explanation.. like I said, google and you will find all kinds of information.
Greg