I downloaded the installer from ATG’s product downloads page which is only available if you have purchased a Standard or Premium Support contract. Previously this page was publicly available but now it is hidden. Fortunately I found the link at ATG’s Google group ATG_Tech where they announced that ATG 2007.1 is available.
ATG Support quickly responded to my email to firstname.lastname@example.org and within a few hours fixed the installer and made available a new installer on the product downloads page.
Lesson learned: Contact ATG Support. They’re good.