I'm currently facing a sandbox issue as well.
I'm working on an update of a released Game Center enabled game, which I tested in sandbox mode before release, of course. So at some point I could login to the sandbox, but not anymore. Every time I install the game on the iPad I am asked to log in to Game Center, but the live one, not the sandbox. It's clearly a developer version, signed with my Developer provisioning profile, and as per Apple's documentation there should only be one case when the real Game Center would kick in instead of the sandbox, and that is if it's a "Signed Distribution build" AND the audience are the "End Users".
I tried all the "tricks" like logging out of Game Center, and deleting the App from the device and reinstalling it. Nothing works, I always end up in the live Game Center (which sucks for testing).
So, who else has or had trouble like this? And how did you fix it, or what else can I look for?