Googled around, searched the nVidia site some more.
Found no way around this mess.

So: thank you Windows XP: I used the System Restore
function to back off the driver update. I'm now happily
back with a monitor-spanning taskbar.

Coupla notes to nVidia:

[1] Folks who run multiple monitors tend to be
non-trivial users. Non-trivial users tend to have
crowded taskbars. Thus, taskbar spanning is
an important feature. It's a BAD THING to just
take it away with no word about it. Makes
users DISTRUST any future driver updates.

[2] Please bring this feature back.

Stan