I didn't discover this - but it was reported to me by a friend and former coworker, Nathan Dintenfass. I'm copying his findings here verbatim:
I was trying to install CF8 on a new MacBook Pro, and I kept getting an error that said "installer user interface mode not supported". I tried updating my java VM, but no dice. Turns out the issue is that the installer app was in a folder containing a "!", and that special character in the path prevented it from loading. Once I moved the installer to a path without a special character it worked.
Interesting! I can't imagine ever naming a folder like that though. Anyone else run into this?