There were lots of discussions about the recently-launched Droid X, and why it seems to get broken when trying to install a custom firmware on it. Motorola came up with an declaration, claiming that these who tried custom firmwares on their phones didn't actually break them, their phones just went in recovery mode by themselves.
What creates the "problem" is eFuse, which is an system designed by Motorola to verify the bootloader which runs on their phone. More exactly, if eFuse detects unapproved software, it will automatically enter the phone in recovery mode. When the user tries to boot with an official firmware, eFuse detects that as an approved software, and lets the phone boot normally.
This is really interesting, I am sure that there will be soon methods of avoiding eFuse (or making it think that custom firmwares are actually approved software).