In BUGtrack 3.60 we've added so anticipated Private Comments, aka Notes functionality.
The list of actions allowed by role has been extended with Note action and corresponding notification setting. When it is on, the member of the role will be able to add the notes through Note button located next to Edit and view own notes as well as to view the notes added by other users. Mass Note action is available as well. When Note action is disabled by the role, the notes are not displayed and the files attached to them are not accessible.
The note allows adding the text and the attachments; no other fields available. This is done intentionaly; since BUGtrack tracks changes made in fields such a priority, type, category or phase, careful user can restore the history of changes. If notes are invisible, it would look like issue properies misteriously change from one value to another.