After some minor hacking (disable email autoconfirm delete), I rebooted the phone from the shell prompt, then disconnected the USB cable. A few seconds later, I noticed the phone had fully shut down. Upon starting it back up, I got stuck in "Signed Out" screen that offered either to reboot (endlessly) or erase all data.

But, thanks to below Palm support forum (with no Palm staff involved), I was able to sign-in to my Palm Profile again.

Re: Stuck In "Signed Out" Loop After webOS Upgrade - webOS Software - Palm Support Community