Microsoft has just started taking actions against Xbox Live users who have inflated their gamerscore through cheats or sharing their account with others. Owners of such accounts will find their scores reset to zero and unable to regain the lost achievements from previous games. Affected players will however be able to unlock achievements through new games.
It might not seem like a big deal but it's a step in the right direction to reign in the wild west mentality that seems to have proliferated across some of the network. Gamerscore is meant to be an indicator of prowess and progress and inflating your score through "friends" or services is not just wrong, it's dangerous. Willingly handing over a Live account to someone is like giving them your credit card; they now have access to everything on your Live ID and can make marketplace purchases on your behalf. Unlike a stolen account, since you willingly gave out your information there is little that can be done legally.
Personally this doesn't have much of an impact on my Live experience (I don't cheat and neither do my friends) but it does show that Microsoft is watching the network. We may see outright bans of particularly offensive players in the future.