I've noticed that in wordpress (surely 2.2, not tested on previous, so please tell me if it works) there's no capability separation for editing post and comments (on their post). Even using the Role Manager Plugin.
I run a lot of multiple authors blogs and sometimes I need only administrators to edit comments (and I'm not the only one).
So I developed this:
http://www.laboratoriocaffeina.it/development/2007/07/20/restrict-authors-access-to-edit-comments-the-plugin.html