Since Outlook does not run file attachments automatically, blocks potentially dangerous attachments by default, does not run code in HTML messages by default, blocks external content (Outlook 2003) etc. etc., the reading pane is very, very safe.
There is always the possibility of some unanticipated Internet Explorer vulnerability, of course, but given the protections outlined above, it would have to be a humdinger to put a virus on your machine.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers