You cannot edit comments once they are posted, as an email is sent to everyone you are sharing the project with noting the comment. You do, however, have a 15 minute window in which you can remove a comment after posting. The same is true with of those that you are commenting with. Here's why we do this:

Obviously you don't want clients having the ability to go back on statements they have made, orders they have said to place, etc. but more than anything, we view this comment thread as your place to document all conversations regarding a project. If you can edit those conversations, then the personal, real-life, dialogue aspect is lost in our humble opinion.