I had an interesting experience this weekend - problems with Rock Band 2 on the XBox 360 where there were none before.
The problem was that the game was locking up when the system was connected to XBox Live. I'm not aware of it being a problem with anyone else, but if a network connection with internet access makes the difference, I wouldn't doubt it.
It's pretty painful when a game starts locking up. Unfortunately, I can prove this. I sign in an account that does not have an XBox Live ID associated with it, everything's fine. If I sign in with one that is Live enabled, and it locks up.
Originally, I thought the disc was buggered because the girlfriend confessed to dropping it. But then, I thought, wait, it ran fine under her profile. I was troubleshooting a hardware issue (it causes problems with our Guitar Hero wireless guitar) before this occurred. It was in the system and working fine. Okay...who was it signed in as? Kristin. So, I signed her in, and the game ran fine. No lock-ups, no stutters, no problems. I log her out and log in as me, it locks up.
The differences between our system accounts are few, but there is a very important one: Mine is attached to an XBox Live Gold account, Rabbi Voody. Hers is not. Clearing the system cache (which has fixed problems with other games such as Ninja Gaiden II) did nothing to fix the issue, so the only way I could play the game is unplugging the network connection. Boohoo, not like I needed it at that moment.
I have had problems with the Rock Band Music Store before where it locked up when I tried to access with a standalone XBox Live program that you can download and use for free. I can only imagine that it's related to that, though as I have no proof. Plus, I didn't have a problem with Rock Band 2 the last time there was a problem. But, the problem lasted for quite awhile. Hopefully this is resolved quickly.
So, you have been warned; if you have an XBox Live-enabled sign-in and are connected to XBox Live, you may have problems trying to play Rock Band 2.