If you're part of the Xbox Rewards program, today might be your lucky day. It seems as though the system is dishing out double, triple, and quadruple amounts of what it should be giving to users today for their normal rewards points. My own account increased 1300 today!
According to Joystiq, Microsoft is currently investigating the issue. I imagine that if you spend the balance, they aren't going to take it back and put you in the red. Let us know if you have been the lucky recipient of the "bank error." If so - you might want to snag your favorite co-op XBLA game before the glitch is rectified! You can check your transactions right here.
Update: It looks like Microsoft has recalled any points that were given accidently, but any purchases made with those points don't seem to be affected.