This was discovered through trial and error, rather than through web searches... so for anyone who's run into this problem, here's what worked.
First, the problem. I recently reset my iCloud password. I was foolish enough to have my account set up for iCloud logins. This... was a mistake. After I changed my password, iCloud pushed a password update to all my computers... a broken update. In other words, I lost the ability to log in or authenticate to anything.
Take note! If you have iCloud login enabled, disable it or risk being hosed like this.
Now the fix.
- You'll need a bootable USB key or flash disk. You can use the hackintosh boot disk creator over at tonymac to create one of these. You may be able to use the recovery partition (hold down cmd-R while starting up)... but this seems to have mixed results. It works on some hardware configurations, but not on mine. Either way, you want to get to the installer/recovery partition boot screen.
- In the Utilities menu, choose 'Terminal'
- Type
resetpasswordwithout the quotes. - Select your boot partition
- Select your account from the pop-up
- Pick a temporary 'recovery password'. This can be anything, you'll only need it for the next ten minutes or so.
- Type in your recovery password twice (into the password and verify fields).
- Click save
- Now the important part. Make sure your computer can not connect to the internet. Unplug your network cables, disconnect your router, go into a faraday cage if you have to. Avoid internet connectivity at all costs.
- Back in the terminal window type
rebootwithout the quotes - Use your recovery password to log in, regardless of what the login prompt tells you.
- Open the User Accounts preference pane, and change your password again, this time to a permanent password. Be sure to unlink the password from iCloud if asked. If asked for your iCloud password, use your recovery password instead. This is your iCloud password as far as your computer knows.
- After you reset your password again, and after you're sure you're unlinked from iCloud, you can connect to the internet once again.