Assuming that the tracked page has an AUTHentication mechanism for users it would be nice for piwik to track the habits of authenticated users ( treating every such user as unique ). The unique tag would not expire for such a user.
The glue between the 2 systems could be done by :
1) passing the auth_user parameter to piwik.php at every tracked page, or
2) setting a AUTH_USER cookie that piwik can read to identify an authenticated visitor
One needs to define better what is meant by "track the habits of authenticated users ( treating every such user as unique )." ; this could mean everything. If it means segmetation by user ID or any other dimension, this is covered in other tickets. If it means having a view of the past activity of a known visitor, this should be covered in Live! plugin. #44