Microsoft will pull the plug on Exchange Server 2003 support in April 2009.
Unless another service pack is released for Exchange Server 2003 (which is unlikely), the mainstream support for the product will end on 14th April 2009.
If you’re still running Exchange Server 2003, its time to start thinking of upgrading to Exchange Server 2007. One year from today, you will not receive support from Microsoft if you face problems on your Exchange 2003 installation, unless you purchase the ‘extended support’ option, which is available till August 2014.
More information on the Microsoft Support Lifecycle website.