How to hide the title field in create content (Drupal 6)?
The Automatic Nodetitles module in conjunction with the Token module is perfect for this.  From the project page:


  "auto_nodetitle" is a small and efficient module that allows hiding of
  the content title field in the form. To prevent empty content title
  fields one can configure it to generate the title by a given pattern.
  
  When the token module is installed it's possible to use various node
  data for the auto generated title - e.g. use the text of a CCK field
  (since 5.x).
  
  Advanced users can also provide some PHP code, that is used for
  automatically generating an appropriate title.