As promised, your Gamerscore will now be worth something. Members of the free Xbox Live Rewards service will have monetary incentive to go after those blip-bloop Achievements in order to earn bonuses. "Contenders" will now earn "a special gift" during their birthday month as long as they have earned 3,000G. But there are other, better gifts for members that reach higher Gamerscore thresholds.
At 10,000G, you'll earn "Champion" status and earn a 1% rebate on your Xbox Live Marketplace purchases every month--in addition to the special birthday gift. 25,000G unlocks "Legend" status and a 2% rebate.
While the amounts are rather paltry, they should add up over time. And remember, Rewards members earn 20MSP every month for taking a survey. Over a year, the percentage point rebate coupled with the surveys should add up to at least one DLC track in Dance Central...
To check out all the bonuses (and sign up), you can visit Xbox Live Rewards.