That did the trick. Thanks for the tip.

Code:
            Dim contentPlaceHolder As ContentPlaceHolder
            contentPlaceHolder = CType(Page.Master.FindControl("ContentPlaceHolder1"), WebControls.ContentPlaceHolder)

            Dim ctr As Control
            For Each s In Str_ArrayList

                For Each ctr In contentPlaceHolder.Controls
                    If ctr.ID = s Then
                        CType(ctr, CheckBox).Checked = True
                    End If
                Next
            Next