I don't know too much about modding the XBox 360 but I have heard that the mod chips are designed to go into "stealth mode" when an original disc is put into the system; this (essentially) makes it improbable that Microsoft can detect that you have modded your system so you can play games on XBox live.
If Microsoft has started banning people who have modded their XBox I can only assume that either their "stealth mode" doesn't work (which I would say is unlikely) or players who have used "backups" to play games on XBox-Live have started to be banned. Overall, if I am right about the cause, I would expect that this would result in online enabled games being seeing somewhat better sales on the XBox 360 compared to other consoles because pirates would be unable to play the games any other way.
