Xbox Live Rewards may be offering rewards for earning Gamerscore.
Xbox Live Rewards' twitter account just announced that a new system to earn rewards based upon achievements will be coming this fall.
I am a current member of Xbox Live Rewards. They offer bonus Microsoft Points for renewing your gold membership and spending points on certain content in the marketplace. I haven't earned too many points (I have earned more through Bing Rewards) as I don't tend to spend a lot of money on digital content. It is unclear at this time how the achievements will be worked into the system and if the rewards will be Microsoft Points, avatar gear, discounts, or something completely different.
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