I finally got IE going this morning and I see the same results there; ParentPageID is undefined.
I didn't see any problems on the Page Settings or Page Design pages. But when I submit from the Page Content page I get a JS error on this line: var navLevel = document.pageSettingsForm1.ParentPageID.value.split("|"); ParentPageID is null in this case. Hope this helps. By the way, I was using FF2.