Lisa Schulte has writen about Exchange 2007 web parts in the MSExchangeTeam website.
Web Part URL formats can be used to access Outlook Web Access web parts contents directly.
Exchange Server 2007 OWA has a new URL format to be used for requests to the /owa virtual directory. Exchange Server 2007 will use this format when forwarding web part requests in the Exchange 2003 format to the Exchange Server 2007 /owa virtual directory.
Read more at source: http://msexchangeteam.com/archive/2006/10/26/429362.aspx