I have an AdditionalPageHead Control in my solution in MOSS,
<Control Id="AdditionalPageHead" Sequence="1" ControlSrc="~/_controltemplates/BinaryJam.MyWebApp/jQueryControl.ascx" />
it works fine in my main pages based on default.master or my custom.master with that delegate control in it.
However the OOTB admin.master also has the AdditionalPageHead Control on it, but my delegate does not appear on these pages.
So what is it about the Admin.Master and Admin Pages that I want this control on (I create my own app specific Central Admin Pages in ~/_admin
So I did consider its something about the path but I can't see whats wrong with "="~/_controltemplates" as that should be valid from both starting locations, and it is used by other items in the Admin side of things.