http://support.microsoft.com/kb/908520

When you reply to an e-mail message in Microsoft Office Outlook 2003, your e-mail signature does not appear in the e-mail message. You experience this symptom even if you configured Outlook 2003 to include an e-mail signature every time that you reply to or forward an e-mail message.
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CAUSE
This problem occurs if you configured Outlook 2003 to use the Attach original message option when you reply to an e-mail message. If the original text of the e-mail message is not included in the reply, the reply header data is not created. In this situation, Outlook 2003 cannot determine when to add an e-mail signature to the e-mail message.

Service pack information
This problem is corrected in Office 2003 Service Pack 3. To resolve this problem, obtain the latest service pack for Office 2003. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
870924 How to obtain the latest service pack for Office 2003