I was just looking through the NotificationMailer code and the trackback method may have the wrong arguments... If the other method signatures are anything to go by it should take (trackback, user). Note that I don't think it's used (yet) anyway since I couldn't find reference to it anywhere else in the code (cut-and-paste jobbie?).

Line 17 of app/models/notification_mailer.rb.

Note: This is in the 5.0.3 code (no option to choose this exact version in the version dropdown when creating the ticket).