Salesforce.com recently implemented the ability for a user to “Enable Create Audit Fields”. In the past, this required a Service Ticket and did not include the Notes and/or Attachment objects. But, with this change these objects are included and it is possible to include the Created and Modified On dates when importing both Attachments and Notes.
Previously, we recommended that Notes and Attachments be imported into the Activity-History section of Salesforce.com. This recommendation was based on several factors. The primary was that there was no way to import the created dates. This is no longer the case and now the only major negative for using notes is that you can’t add user fields, can’t create reports or do mass deletes for these object. However, if there are a lot of regular history items, sometimes the Notes can get lost and are better suited in their own object.
More information for implementing this is included in the Following Knowledge Articles
000213290 “Enable Create Audit Fields”
000232909 “Enabling Create Audit Fields for Professional Edition”