I'd like to see this implemented generically (in CPAN.pm as an option) rather than on a per-module basis. Look at the Debian Popularity Contest for a system that tracks usage of Debian modules. This is also opt-in, but I'd rather the CPAN version not be implemented as a daemon. PopCon appears fairly successful, but yeah, I'm not running it on my box -- so take that as you will. Definitely some good examples to read up on though.