I know this has been blogged about on a number of occasions so apologies to those for which this is not news.

As I’m sure everyone is aware Windows 2008 is installed with 128-bit IPv6 support alongside 32-bit IPv4 but can actually run IPv6 exclusively.  …but not if you want to put Exchange Server 2007 on it however.  To quote Technet;

“IPv6 is only supported in Exchange 2007 SP1 when it is installed on a Windows Server 2008 computer that has both IPv4 and IPv6 enabled. If you disable the IPv4 protocol, Exchange 2007 SP1 can't support IPv6.

..and by the power of live search.

The installation of the Exchange Server 2007 Hub Transport role may be unsuccessful on a Windows Server 2008-based computer
Outlook 2007 supports Internet Protocol version 6 (IPv6)
Outlook Anywhere Client Connectivity Issue Because of TCP/IPv6

IPv6 Support in Exchange 2007 SP1
IPv6
Support WebCast: Introduction to Internet Protocol Version 6

[..and for the proper techies amongst us have a look here for the specification - http://www.ietf.org/rfc/rfc2460.txt]