What does the Microsoft Exchange Service Host service do?
When you deploy an Exchange 2007 Mailbox server role, or Client Access role, you get (amongst quite a few other services) a service called "Microsoft Exchange Service Host". Looking at the description, it will tell you that this service "provides a host for server Microsoft Exchange services".
A search in TechNet reveals a bit more about this service (as taken from:http://technet.microsoft.com/en-us/library/aa998342.aspx)
"Configures the RPC virtual directory in Internet Information Services (IIS), and registry data for ValidPorts, NSPI Interface Protocol Sequences, and AllowAnonymous for Outlook Anywhere". The service is only dependent on the Microsoft Exchange Active Directory Topology service.