Ms Exchange Team Blog has just posted this article :
UDP Notification Support Re-added to Exchange 2010
There, you can read :
" I am very pleased to let you all know that in response to your feedback we are adding support for User Datagram Protocol (UDP) notification functionality back into Microsoft Exchange Server 2010. While Outlook 2010 is the best way to experience all that Exchange 2010 has to offer, you have been very clear that many of you have longer client deployment cycles or are waiting for a desktop hardware refresh before upgrading clients. Regardless of the reasons, those of you who have users running Outlook 2003 have seen those users' Outlook performance degrade with the removal of UDP. You can learn more about this issue in the related Microsoft Support article. "
The mentioned article is :
In Outlook 2003, e-mail messages take a long time to send and receive when you use an Exchange 2010 mailboxhttp://support.microsoft.com/kb/2009942
So being, with this update, Outlook 2003 will work faster when using Exchange Server 2010.