Well fuck this, it won't let me reconnect once I'm in the game. I had already deleted my compatibility packs before I read that so I can't use my costume parts right now.
*And it isn't just SC, I can't get into Borderlands either. WTF Microsoft!!!
Korn try this: Delete the DLC off your hard drive, then try and sign in.. It should help the problem, and you can always re-download the DLC after. If not install the disc to your hard drive, that at least helped when I kept getting disc-read errors for MW3.