Turns out that the simple act of leaving any kind of disc in the tray during the ethernet upgrade process apparently can cause both the Toshiba HD-A1 and HD-XA1 models to stall during installation, then subsequently "lock" the machine on the "Welcome" screen. Once the bug is activated, the player is no longer operable and must be sent back to Toshiba for repair.
The only exception is when an actual firmware upgrade install CD is placed in the tray during installation, in which case the bug is not activated.
Note that this problem has also been reported on all previous firmware updates issued by Toshiba, so precautions must always be taken to not leave a disc in the tray when installing any upgrade via ethernet, regardless of version.
The bug was first reported in this detailed thread at AVS Forum, where users have since begun offering various helpful tips on how to avoid the problem.
Apparently, Toshiba has also been made aware of the bug, and has been offering to repair players rendered inoperable during installation free of charge. However, no official word has not yet come if and when Toshiba will fix the actual download code itself.
In the meantime, should you have already experienced problems with the firmware upgrade, or suffered a complete lockdown of your unit, you can contact Toshiba directly via their official website.