While it's the same release date as Halo3 beta, it seems that people who didn't download Halo3 also get banned. They probably detect backup discs (and not the modified firmware) when you play 'em online (not confirmed yet).

Even people using the new firmwares with disc-jitter added get banned, so it looks like Microsoft found another way to detect it. However we also see reports of people with modified firmwares that are not banned (yet). Microsoft is known to ban with delay though, so it'll probably take a while before we got a good view on who got banned for what reasons.

Just like on Xbox1 it looks like Microsoft bans the console unique ID (serial), not the LIVE user. Microsoft will not allow anyone to login on LIVE on a banned console.

Many people on the official xbox.com forums also claim they got banned but didn't have a flashed firmware ... we'll have to see how that story develops in the coming hours and days.