I just got banned as well after halo 3 beta... I am using an original store-bought retail disc of crackdown, downloaded the halo 3 beta using it, got to play for about 3 hours then banned.
I am also not using a modified dvd-drive or firmware hack at the moment. I did used to use a hacked Hitachi 47 drive with FW 2.3 before the Spring update. Admidst all the ban talk I decided to flash back to original firmware and not use back-ups for a while. I did this before the Spring update.
I now get the banned message when doing a network test (note when you do a network test you only see the banned message in "More Info" under Network Adapter or IP, not under Xbox Live).
This means that either MS have been loggin poeple playing back-ups for a while, even before the spring update, or there are indeed false bans happening as some people are reporting. I know there were a lot of false bans with GoW when that came out, and poeple were let back on after a few days. I hope that's the case this time for me!