For the rest of October, if you spend a certain amount of Microsoft Points on Xbox Live stuff, you'll be given points in return, which you can then spend on more stuff.
Specifically, if you spend 2400 points on anything - be it games, downloadable content or a helmet for your avatar - you'll get 800 points dumped back into your account.
The catch? It won't be dumped straight away; you'll have to wait until the end of November for the points to magically appear. Oh, and the deal won't be running in Italy or France.