Under System Preferences / Users & Groups / Login Items I can see all of the items that I've selected to launch at startup, but the folder,
Library/StartUpItems is empty.
Where can I see these items?
The file the preference pane loads the list of items from is:
It is a binary file. You can use Xcode to open it.
For example, if you add iPhoto to your login items:
com.apple.loginitems.plist will look similar to this: