I was asked at the Windows Intelligence event I presented at on Monday if there were specific requirements on what version of Windows 7 could be used with BranchCache. It turns out there are, you can only use Windows 7 Enterprise and Ultimate Editions to take advantage of BranchCache - even if you are doing a workgroup configuration and not a domain.
The step by step guide has the requirements for both the client and the server and can be found here.
It appears that Microsoft is assuming small and medium sized companies should be using Enterprise or Ultimate Editions to take advantage of newer technologies like BranchCache and DirectAccess. I will ask around to see if this will be changing at all in the future and post back up what I hear. I am a bit surprised to read that Windows 7 Professional didn't make the cut for BranchCache as a feature but licensing is a complicated thing that honestly I think very few folks truly understand, even within Microsoft.