From http://www.vmware.com/beta/server/server2_beta_faq.pdf:
"Support for VI API 2.5: Allows VMware VirtualCenter to manage both ESX
Server and VMware Server 2.0 hosts plus support for VI SDK and VI Perl.
This will enable an easier transition path for customers who upgrade
from Server to VMware Infrastructure, or comprehensive management for
customers who use both VMware Server and VI in their IT environments"
Lars