There have always been a couple of public FTP servers you could get backups of someone's original TiVo hard drive files. Typically, you'd go to it if your TiVo hard drive died. You could buy a new drive, restore the original files from the backups, plop it in your TiVo, and use it.
It's always been kind of shady -- while you couldn't steal service or anything, it was hosting a copy of the complete TiVo software for anyone to grab. People have been pretty hush-hush about it for the most part, but according to Slashdot, people hosting backups are now being sent cease and desist letters (here's a copy of the letter's text).
I've watched the TiVo hacking community for a while, and I think the real tipping point on TiVo suddenly caring about backups might be the recent hacking work done to get the 4.0 OS on series 2 DirecTiVos. Thanks to these freely available backups, people could grab the 4.0 OS from a standalone TiVo, and restore it on a DirecTiVo system. People have also been trading hacked backups of series 2 DirecTiVo OSes with the USB ports and networking added. This is my best guess, but I'm betting DirecTV raised a stink about it and forced TiVo to do something about these previously innocuous backups. [thanks Mary and Andy]