Well I was chatting with a buddy who got banned too. Here's how it happened to him:
He was using Xbox Media Player, his mod chip was on (obviously) and his network cable was plugged in. He hit the wrong button, got sent to the original Xbox Dashboard. According to him, the Xbox dashboard checks for mod chips, and flags your "mod status" so that the next time you connect to Live, it tattles on you. So he got banned. Simply by being in the Xbox dashboard with the mod chip enabled. He didn't try to connect or initiate any "Live" activity.
I know my current setup only boots to the original Xbox dashboard when the chip is OFF, but there seems to be a few places in EvoX (and my installed homebrews) where it might dump me into the Xbox dashboard. So I tread lightly, as I get to know the terrain better.