Neither Windows 2008 nor SQL 2008 is supported by OCS 2007 R2. But if you really want to do it, here are some tips:
Use SQL 2008 R2
SQL 2008 will fail to install on Windows 2008 R2. Instead of trying to 'fix' it, you may just use SQL 2008 R2.
Pool Creation Failure
a) manually create database rtcconfig (assuming you know how to use SQL Management Studio)
b) manually run the script on OCS installation CD.
D:\Setup\amd64\DbSetup>cscript.exe poolcfgdbsetup.wsf /clean /sqlserver:rcdn /serverrole:EP /verbose
c) continue GUI install
Error: "Not available: IIS 6 Management Compatibility and IIS Windows Authentication role services must be installed before you Deploy Server."
Solution: Install all IIS6 compatibility options. Install all authentication options
Error: "The Windows Media Format Runtime is required in order to install this component. Installing the Windows Media Format Runtime may require a system restart to complete the installation. Click OK to continue with the installation."
Solution: install the Desktop Experience Feature.
Error: "[0xC3EC78D8] Failed to read the Office Communications Server version information. This can happen if the computer clock is not set to correct date and time."
Solution: Uninstall MS Crypto API security update KB974571