OK -- we found something. If this is the only thing, it's an easy fix.
Turns out that in 6.5, the submit button on the registration form is named "submit" In 6.7.2, it's named "submit_form" (This is in Templates/public_register_form.pl)
I can certainly hack it back so that the submit button is once again named submit -- but does that ring any other bells with anyone for anything else down the road?
Thanks.
Marianne