Major Nelson has posted a warning on his blog to all the folks out there thinking about sharing their Xbox Live accounts or using altered game saves to cheat at Gamerscore. In short, don't do either one. Major Nelson uses the example of giving someone your account so they can gather all the skulls in Halo 3 for you. If said person were to download XBLA games or movies onto that account, there is basically nothing Microsoft can do for you. The only recourse is for Microsoft to cancel the account altogether, and you'll still be out any MS Points spent by whoever you gave your account to.
Major Nelson also lays down a warning to anyone using artificial means to increase their Gamerscore. He notes that such accounts may have their gamerscore removed altogether and any achievements earned on such an account will not be able to be reacquired. And, as you might expect, such an account (and the console on which it resides) could also be banned from Xbox Live entirely.
We're not sure what prompted the warning (an increase in one or both activities perhaps), but we're sure it's good advice. Besides, what's the point of cheating to get Gamerscore? Where's the fun in that? Remember kids, if you cheat to inflate your Gamerscore, you're no better than a pro athlete.