Couple of ways to get your hands on Windows Server 2008 R2 RTM or play around with it.
180 day Evaluation version
Evaluate Windows Server 2008 R2 free for 180 days. Web, Standard, Enterprise, Datacenter and Itanium images are available for English, German, Japanese, French, Spanish and Chinese languages (Itanium is only available for English, German, Japanese and French).
Virtual hard Drive (VHD) Windows Server 2008 R2 Enterprise and Server Core
The Microsoft VHD file format specifies a virtual machine hard disk that can reside on a native host file system encapsulated within a single file. The format is used by Virtual PC 2007, Virtual Server 2005 R2 and Hyper-V and all future versions of Microsoft Windows Server that includes hypervisor-based virtualization technology.
Windows Server 2008 R2 Enterprise and Windows Server 2008 R2 Enterprise Core Server VHD files are now available for download.
Test Drives (Windows Server 2008)
Try out Windows Server 2008 in a virtual environment by following step-by-step guides that walk you through specific scenarios around running applications from anywhere, high availability, server management, and security and policy enforcement. (Windows Server 2008 R2 Test Drives will be released Q4 CY2009)
In these TechNet Virtual Labs, you'll have the opportunity to test-drive new and improved features and functionality in the Windows Server 2008 release, including management, network protection, and improvements to Terminal Services. Each lab takes approximately 45-60 minutes to complete. (Windows Server 2008 R2 labs will be released Q4 CY2009)
TechNet Plus and MSDN subscribers can also get access to RTM.