Yesterday (12th July 2010) at the Microsoft Partner Convention, Microsoft made publically available the beta of Service Pack 1 for Windows Server 2008 R2 and Windows 7 (in English, French, German, Japanese and in Spanish).
Microsoft for sometime have stated that the only major new features in Service Pack 1 are Dynamic Memory and RemoteFX
Dynamic Memory allows for memory on a host machine to be pooled and dynamically distributed to virtual machines as necessary. Memory is dynamically added or removed based on current workloads, and is done so without service interruption.
Microsoft RemoteFX in Windows Server 2008 R2 SP1, a new set of remote user experience capabilities that enable a media-rich user environment for virtual desktops, session-based desktops and remote applications is introduced.
This document published on Microsoft.com details the other changes.
Windows Server 2008 R2
Enhancements to scalability and high availability when using DirectAccess
Support for Managed Service Accounts (MSAs) in secure branch office scenarios
Support for increased volume of authentication traffic on domain controllers connected to high-latency networks
Enhancements to Failover Clustering with Storage
Additional support for communication with third-party federation services
Improved HDMI audio device performance
Corrected behaviour when printing mixed-orientation XPS documents
Both Windows Server 2008 R2 and Windows 7
Change to behaviour of “Restore previous folders at logon” functionality
Enhanced support for additional identities in RRAS and IPsec
Support for Advanced Vector Extensions (AVX)
Updates Remote Desktop Protocol (RDP) to version 7.1
The Service Pack also includes 473 Updates for Windows Server 2008 R2 and Windows 7 of which only 17 are security updates.
These are detailed in full in the spreadsheet
Hot fixes and Security Updates included in Windows 7 and Windows Server 2008 R2 Service Pack 1 Beta
which is located in the Documentation for Windows 7 and Windows Server 2008 R2 Service Pack 1 Beta (KB976932) that was mentioned earlier.