The team says:
"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.
We've heard you loud and clear and so this functionality will be re-introduced in Exchange Server 2010 Service Pack 1 Roll-Up 3 (SP1 RU3). As a result, we have adjusted the RU3 schedule in order to complete the engineering work required to include UDP support within the rollup, which we now expect to release in March."
Source: Ms Exchange Official Blog.