I am involved with delivering Exchange Unified Messaging training to MS partners around EMEA. In getting prepared, I see that the Exchange team's blog has an article about Exchange 2007 SP1.
One neat feature in Exchange 2007 SP1 is proper IPV6 support - On Longhorn Server, Exchange 2007 SP1 supports native IPv6 networks. There are a host of other updates, including updated the Exchange management console with features that did not make RTM.