This *is* possible, at least in version 7, but as rcollyer supposed there is a Global security option which cannot be set from within the Notebook (automatically, that is).

First change the Global option:

    SetOptions[$FrontEnd,
      GlobalInitializationCellWarning -> False
    ]

Then in the Notebook you need to evaluate:

    SetOptions[EvaluationNotebook[],
      InitializationCellEvaluation -> True, 
      InitializationCellWarning -> False
    ]

Or use the **Option Inspector** in either case.