The Exchange Product group does a great job of summarizing the most interesting changes and improvements in Exchange 2010 SP1 in this post on the MS Exchange Team blog. You can also find a more comprehensive list of the changes and improvements included with this latest service pack in the "What's New !" section in the Exchange 2010 TechNet documentation. 

Microsoft

• What? A Blog Post? Tiered Storage? (Ian Hameroff's blog)
• Lights, Camera, Archiving! (Ian Hameroff's blog)
• First Details of Exchange Server 2010 Service Pack 1 Available! (Scott Schnoll's blog)
• New High Availability Features in Exchange 2010 SP1 (Scott Schnoll's blog)
• Exchange 2010 SP1: Database Integrity checking (Exchange Team blog)
• Exchange 2010 SP1 FAQ and Known Issues (Exchange Team blog)

Community

• Manage Public Folders using the new Manage Settings Wizard in Exchange 2010 SP1 - TechTip 1 (Henrik Walther's blog)
• Personal Archive improvements in Exchange 2010 Service Pack 1 - TechTip 2 (Henrik Walther's blog)
• Setting Static IP Addresses for a DAG using the GUI in Exchange 2010 SP1 - TechTip 3 (Henrik Walther's blog)
• Exchange 2010 SP1 Retention Policies (Elan Shudnow's blog)
• A quick look at Outlook Web App improvements coming in Exchange 2010 SP1 (Steve Goodman's Tech Blog)
• Calendar Sharing Improvements coming in Exchange 2010 SP1 (Steve Goodman's Tech Blog)
• New OWA themes available in Exchange 2010 SP1 (Steve Goodman's Tech Blog)
• A look at the Import/Export Mailbox improvements in Exchange 2010 Service Pack 1 (Part 1) (article by Henrik Walther)
• What’s New in Administrator Audit Logging in Exchange 2010 SP1 Beta? (Mike Pfeiffer's Blog)
• Performing Maintenance on DAG Members in Exchange 2010 SP1 (Mike Pfeiffer's Blog)
• Set up Federated Free/Busy and Calendar Sharing between Exchange 2010 SP1 and Outlook Live (Steve Goodman's Tech Blog)
• Exchange UM Auto Attendant Key Mappings Not Transferring Calls After SP1(Microsoft UC Made Easy-Randy Wintle)
• DAC: Changes in Exchange 2010 SP1 (EighTwOne - Michel de Rooij)
• RBAC Overview Sheet including SP1 changes (EighTwOne - Michel de Rooij)
• Exchange 2010 SP1 Schema Version info (EighTwOne - Michel de Rooij)
• Exchange 2010 SP1 RTM info, including changes compared to SP1 Beta (EighTwOne - Michel de Rooij)

International

• Exchange 2010 Service Pack 1 What's New, in Russian (Arman Obosyan's blog)
• Exchange 2010 Service Pack 1 Overview, in Russian (Arman Obosyan's blog)
• Exchange 2010 SP1 Hierarchical Address Book, in Russian (Arman Obosyan's blog)  
• Changes in Outlook Web App 2010 SP1, in Russian (Oleg Krylov's blog)

Download Evaluation Version

The Exchange Server 2010 SP1 (version 14.01.0218.015) has now been released to the public and is available for download here.

You can find the public announcement from the Exchange product group here.

It's recommended to use Use Exchange Server 2010 SP1 Help">Exchange Server 2010 SP1 Help and release notes to help you plan, deploy, manage and upgrade to Exchange 2010 SP1.