EA sport series 2005,
Star Wars Battlefront
and probably some more
Many, many ppl using those games has got banned. I think that M$ has learned something and even the newest X2 bioses don't help to prevent connecting to XBL servers. Disconnect the network cable from your console when playing the newest, backed up live titles and you'll not get banned.
It seems like these games connect automaticaly, without any user's permission. They can ask you to sign into the service and you can disagree, however they do connect to XBL and spy your console, if the plug is in.It also seems like those games do automatical search for XBL while connecting to another Xbox thru System Link option. Why that? Because M$ knows that there are many ppl that use XBC/Xlink to emulate System Link and that they mostly use pirated games, playing on these. If the Xbox succesfully connects to the XBL servers, while trying to connect to another Xbox, it sends all the needed "ban data" to the server. Voila, so you get banned playing on XBC.
But these are only my stupid feelings about it

. Report if you haven't used any of the new XBL titles and you've got banned. It may be a clue, that my theory is mistaken or correct.
EDIT:
I have written this post, cause i were sick of hearing about LPC/d0/HDD detection ;P. See that the problem has started right after the wave of those new games and search the reason here.