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Updated to include link to SP1 release notes.  

The eagerly awaited service pack 1 for Exchange 2007 has been released.
In a change from service packs for earlier versions of Exchange, you can install Exchange 2007 fresh from this download - therefore the download files are quite big.
Available in both 64 bit and 32 bit, although remember that 32 bit is not supported for production use. Evaluation only.

From the download page:

Overview

Microsoft Exchange Server 2007 Service Pack 1 (SP1) has been designed specifically to help meet the challenges of any business and the needs of all the different groups with a stake in the messaging system. Exchange Server 2007 SP1 is a mission-critical communications tool that enables employees to be more productive and access their information anywhere and anytime while providing a messaging system that enables rich, efficient access to e-mail, calendar items, voice mail, and contacts. For the administrator, Exchange Server 2007 SP1 provides advanced protection options against e-mail security threats, such as spam and viruses, as well as the tools to help manage internal compliance and high availability needs.

In Exchange Server 2007 SP1, several new features and improvements will extend the Anywhere Access capabilities of Exchange Server 2007 to help make employees more productive on whatever device they’re using, provide additional Operational Efficiency tools for administrators seeking a streamlined management and deployment experience, and enable advanced Built-in Protection for more robust high availability and compliance scenarios.

Improvements in Exchange Server 2007 SP1 include:

Anywhere Access

• Integrated Exchange Unified Messaging functionality with Microsoft Office Communicator 2007 and Microsoft Office Communications Server 2007.
• Outlook Web Access additions, including public folder access, S/MIME support, personal distribution lists, and mailbox rules editor.
• Webready document viewer supports Microsoft Office 2007 documents in addition to Microsoft Office 2003 documents.
• Extended language support in Outlook Web Access with Arabic and Korean spell checking.

Operational Efficiency

• Support for Windows Server 2008 deployments, including benefits in flexible clustering, native virtualization, advanced networking, and simplified management.
• Additional tools in the Exchange Management Console, including public folder management and configuration options for clustering and POP/IMAP access.
• Improvements to the Exchange Management Shell syntax and import-export PST in the move-mailbox command.
• Wider variety of web services for application development, including public folder access, delegate management, and folder level permissions.

Built-in Protection

• Addition of Standby Continuous Replication (SCR) for site resilient high availability deployments.
• Extended Exchange ActiveSync policies for mobile policy enforcement.
• Information rights management pre-licensing by the Hub Transport role.
• Secure Real Time Protocol (SRTP) support in the Unified Messaging role.
• Support for IPv6 when using Windows Server 2008.

This is the link to download it. However you need to register, using a Microsoft Live ID:

http://www.microsoft.com/downloads/details.aspx?FamilyId=44C66AD6-F185-4A1D-A9AB-473C1188954C&displaylang=en

UPDATE:  

At the time of writing, the link to the release notes takes you to the RTM release notes, not the SP1 release notes. To see the SP1 release notes (gives you something to read while it downloads) go here: http://www.microsoft.com/downloads/details.aspx?FamilyId=5770BD59-376E-42EC-B940-BE6225CD97FF&displaylang=en