"Windows Server 2008 Datacenter licenses include unlimited virtualization rights, meaning customers have the use rights to run an unlimited number of virtualized instances of Windows Server on processors licensed with Windows Server 2008 Datacenter without purchasing additional licenses"
I'd talk to your Microsoft reseller but that doesn't read as if you can install any old OS on there. You need at least one existing license per OS. That said, our Server 2008 Ent purchase also included 2003 R2 keys as well (similarly licensed) so I'm surprised you didn't get 2003 R2 keys.
Reading more about it: it means you can have as many Windows-server based VMs running w/in 2008 Datacenter as your box can physically handle. But, you need the appropriate licenses for those operating systems.
Q. Say I have a Windows Server 2008 Enterprise license assigned to my server. What if I want to add a fifth instance of the server software in a virtual operating system environment?
With a license for Windows Server 2008 Enterprise, you may run up to four instances of the software in virtual operating system environments. If you want to run a fifth instance in a virtual operating system environment, you must acquire and assign an additional license to that server. It may be more economical to license the server with Windows Server 2008 Datacenter or “step up” your Enterprise license to Windows Server 2008 Datacenter.