With the amount of posts we see on our forums ... it's clear, Microsoft decided to start banning Xbox360 with modified firmwares today (both Hitachi-LG and Toshiba-Samsung drives).
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.

Update: This was just posted on Microsoft's Games Global Marketing team blog:

[QUOTE]

One of the great things about LIVE and the unified community is that we work hard to create a level playing field for all gamers and have a no tolerance policy towards inappropriate behavior like hacking or cheating. As part of our commitment to our members, we do not allow people that we have detected to have modified their console to connect to LIVE. This is an important part of our efforts to try and maintain a fair gaming environment for the large majority of gamers that play by the rules. This topic is more important than ever given the recent release of the Halo 3 beta. As a result, some consumers that try to login to LIVE who we detect have illegally modified their console will get an error code (Status Code: Z: 8015 - 190D) when trying to connect to the service. These users will not have their account automatically banned from LIVE, but they will no longer be able to access the service from the console they modified.



We have stated in the past that customers can only enjoy access to the Xbox LIVE community through the use of a genuine, unmodified, Xbox console and we will continue to enforce this rule to ensure the integrity of our service, the protection of our partners and the benefits of our users.

[/QUOTE]"