Let me tell you what I know (wich is not much {grin}):

• PPM is maintained by ActiveState.
• AFAIK - at least on Windows plataforms - PPM explicitly install packages from a list of pre-registered repositories. Since it uses it's own file format (.ppd), you can't install modules directly from CPAN, since the majority of them (the ones I've seen at least) are distributed in another format (the well known extract-configure-make-make install routine).
• There are some CPAN cases (specially Win32 modules) where the author gently posts a URL where you can download the .ppd file.

Regards,