Kotaku linked me to the xbox-scene forums, and that's where you get the collections of guys that got banned, since it's a site about hacking your console. Basically it looks to be one of two scenarios, or both:
1. The games each have a unique serial code built in and when they get pirated that one copie's serial code gets repeated onto each new download, meaning that every 360 that uses this game code would get banned. Microsoft basically scans everyone's games(I'm not sure if this would be legal) and they weed out these repeated codes and ban the 360's they are on.
2. Microsoft purposely planted the uncharacteristically early Fallout 3, Saints Row 2 and Gears of War 2 releases onto torrent sites to bait people into downloading them and catching them that way like it were inked money from a bank robbery.
It seemingly doesn't follow a pattern which has people believing that MS isn't banning every offender, but is doing them in groups to cause a hysteria among the 360 owners with custom firmware. The conspiracy at the forum is they are doing it purposely each Fall(this happened last year too) to promote these people to buy a new 360 as part of Christmas to replace their old one, which is unfixable and can never go online again.