One area we always seem to need to brush up on is Microsoft Licensing.
A site that has the necessary documentation:
We are looking to find out whether a client of ours can install Windows Server 2008 x64 Server Core with the Hyper-V role enabled and virtualize a Windows Server 2003 Standard server using the 1+1 virtualization rights plus downgrade ability with Open Licensing.
This is the site we were pointed to with the specific document:
Pages 24-25 explain the Server Standard 1+1 and Enterprise 1+4 licensing guidelines quite well.
So, the answer is yes we can run Server 2003 Standard with their Open Licensing server license.
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book