OVERALL STATUS: No, having Oulook 2003 clients is not a deployment blocker. However, you need to understand the following sections and make configuration changes as applicable.
Back since November 9th, 2009 where Exchange Server 2010 released to manufacturing (RTM), there have been a growing concern around whether enterprises are prevented from upgrading or migrating their current Exchange 2003 or Exchange 2007 based messaging infrastructure to Exchange 2010, if Outlook 2003 clients is used within the organization.
But in this article includes are a few additional concerns about Exchange 2010 and Outlook 2003
Exchange 2010 lack support for UDP Notifications
Exchange 2010 Exchange Server name appears as Instance – <GUID>
Exchange 2010 & Outlook 2003 Offline Address Book (OAB)
Exchange 2010 RPC over HTTP Connectivity
Exchange 2010 Opening multiple shared calendars & additional mailboxes
Exchange 2010 RPC Encryption Requirement
Exchange 2010 Public Folder Database requirement
Special Thanks to Henrik Walther