I totally agree with you that it needs a single sign on (maybe CPAN could provide an OpenId account?) - but I don't think that requires centralisation.
well, if you have a module on CPAN, you'll get emails directly to your cpan address, you will get emails from RT (people even post bug reports on annocpan where they don't belong), to check out the ratings you have to check out the web page, to check out the discussion forum you have to check the web page (or subscribe to get postings per email? haven't tried it out), to get the latest tests you subscribe to an RSS feed (this is something i like).
there is no pointer to how to set up a gravatar (mine has worked for a couple of days before it vanished, and i don't know why).
don't get me wrong, it's nice that there are many tools but there are kind of thrown together, and with gravatar as an example, poorly documented (in the orignal posting on perlbuzz it says 'create a gravatar-account with your cpan-address'. which one? the private or public address? i had to try out both. where can i ask?)