When I first heard about the integration of Microsoft System Center Virtual Machine Manager (SCVMM) 2008 and VMware's ESX Server, I got really excited because I thought that SCVMM could directly manage ESX. After some research, I discovered that my assumption was incorrect.
SCVMM integrates only with VMware vCenter (formerly VMware VirtualCenter). As you might know, at least one vCenter server is required to manage most of the enterprise features of VMware Infrastructure, such as High Availability (HA), vMotion, and Distributed Resource Scheduler (DRS). SCVMM communicates with a set of published vCenter APIs. For SCVMM to integrate with ESX, you must purchase vCenter (at a price of approximately $6,000). But if you already have vCenter, you can use SCVMM to manage a mixed Hyper-V and ESX Server environment.
SCVMM is significantly less expensive than vCenter. So if in a future release Microsoft could incorporate all of vCenter's features in SCVMM, the company would have a very attractive product that would significantly reduce the cost of implementing the enterprise features of Virtual Infrastructure. For now, we'll have to wait and see whether SCVMM will be a suitable replacement for vCenter in the future.