I am also seeing this problem.

My store, http://www.performsafe.com/, is running v1.3.9d with authorize.net payment module.

It's been running fine, and suddenly a couple of days ago customers started reporting this problem.

NOTHING HAS CHANGED! Yes, I know everybody says that, but I am the only person with admin access to the site, and I haven't changed anything.

I have no idea how to even try tracking this down.