One question I see asked a lot on our internal forums and by customers is what do I miss if I stay with Exchange Server 2003 when implementing Office Communications Server 2007.
You don't actually "lose" any functionality from either of the products by staying on one version or another, however you do "miss" the opportunity to provide a much tighter integration between Exchange and OCS and a much better experience for your user base by opting to stay with Exchange 2003.
Exchange Server 2007 introduced the Unified Messaging role as a component of it's architecture. Designed to be complementary to the telephony capabilities of LCS2005 SP1 and later OCS2007 the UM role in Exchange provided amongst other things the capability for a users to receive voicemail in their inbox direct from the Exchange Server rather than within a PBX infrastructure.
This concept of anywhere, anytime access allows companies to direct unanswered calls to the Exchange infrastructure and then delivers the voicemail to the users inbox allowing them to see their missed calls when they login next or from their Windows Mobile device - and then play the message direct from their client and/or on a phone if that is the preferred option.
Outlook Voice Access was also introducted with Exchange Server 2007 and is beneficial for those users whom are out of the office or on the road a lot - allowing them to connect to and manage their inbox, calendar and personal contacts from any telephone either via natural speech (english only today) or a touch tone interface.
More information on UM and OVA can be found here:
http://www.microsoft.com/exchange/evaluation/unifiedmessaging/umcommunications.mspx