promised, PS3 firmware update 3.21 has gone live -- and no, it's not some kind of elaborate April Fool's joke crafted by Sony. Yes, the update removes Linux support from Phat PS3 systems. And yes, the update makes it explicitly clear that this update will remove a feature (and corresponding menu options) from your system.
This is a mandatory update, meaning you won't be able to log on to the PlayStation Network without it. So, might as well get it out of your way as soon as possible. You can use either System Update from the XMB, or download the update from PlayStation.com to a USB stick.
[Thanks to everyone that sent this in!]