From eurogamer.net:

Credits earned by gamers before Halo: Reach's official launch will be removed, Bungie has warned.
Bungie will reset Credits earned and any gear bought ahead of the Xbox 360 exclusive's 14th September launch.

"Some of you are playing Halo: Reach a little bit early," communications man Eric Osborne wrote in Bungie's forum.
"It happens. With millions of copies out of manufacturing quickly making their way into the massive global retail chain, it's not terribly uncommon for a few discs to 'accidentally' slip into a few lucky customers' hands. Que sera."
Full Story: eurogamer.net