I've not received any official word from Microsoft on the banning of modded Xbox consoles from Xbox Live yesterday but this on Major Nelson blog makes it clear that the ban was deliberate:
As you can imagine, we have a zero-tolerance policy when it comes to modded consoles connecting to LIVE.
We're very serious about this, and if we detect you have a modded Xbox then you will not be able to connect to Xbox Live. If you see this screen and get Status Code: Z: 8015 - 190D…that means your console has been banned.
We'll continue to enforce this rule to ensure the integrity of the service and protect our partners and users.
To the members of our community who play by the rules, thanks and we’ll see you on LIVE.
Microsoft has every right to do this but I am left wondering how many legitimate users not using modded Xbox 360 consoles have been kicked off Xbox Live? I've heard from one user today who claims that his console was bought brand new and not modded yet he's now unable to access Xbox Live.