Unfortunately development on modules which allow users to comment on profiles of other users is still very much ongoing.
At the moment, the most suitable working module is the guestbook module, as this provides each user with their own guestbook as part of their profile which other users can comment on. At the time of writing, this module is still very much "in development", but is still the most suitable for now.
There are however a number of other modules under development which are more suited to this type of feature. The most promising is a Facebook-style statuses (microblog) module (http://drupal.org/project/facebook_status). There is no Drupal 7 version available, however there is a discussion about how this is going to work at http://drupal.org/node/671822.