XBL has finally decided to put gamerscores to good use. as a result they will begin rolling out a rewards program though atm its very crude
for those with 3k - 9,999 achievement points you will recieve a special gift on your official birthday.
those with 10k - 24,999 you are given a permanent 1% off discount on XBLA titles, DLC, games on demand, avatar shwag, etc..
those with 25k+ you get a permanent 2% off any XBLA purchases.... (see list above for 1% reward)
you may not however use your 1-2% off discount for purchasing gold status memberships or renewing it