A logged in user registering for an event should have his registration form pre-filled with all appropriate information. I created a stock drupal install (no modules other than SMTP) and installed civicrm 1.7 build 9184. Created two custom data groups with different field types. Then created two profiles, one for all options (registration, profiles, search, CiviEvent) and one for participants. Created a user using this profile. Created an event that uses both the registration profile and the participant profile. Logged in to Drupal and then tried to register for the event. Nothing is pre-filled.
I duplicated this on sandbox (profiles are: Elliott's Guests and Participant Event Data, Event is WRC Reception, user is houdini).