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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:

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

Or use the Option Inspector in either case.