I just setup email to case and can open cases just fine.  I have an auto-resposne rule that notifies the contact of their case ID.  The problem is that when I send a reply email with the case ID in the subject line of the email the Email to Case app is creating a new case instead of attaching to an existing case.  Does anyone know what the algorithm is for associating an email to a case?  SF.com support was confused and had me try a bunch of additional fields in the subject and body of the email (Case Number, Case ID, Reference ID) but nothing seems to work.  Has anyone else run into this problem?

Thanks,

Mark