I received a great question via my blog from Mitchell Land about whether or not IIS must be enabled on the Data Tier of a Team Foundation Server installation. The short answer is no, you need to enable IIS 6.0 w/ ASP.NET on the Application Tier but not on the Data Tier. The Application Tier makes use of ASP.NET Web services to communicate with Team System clients. I believe IIS 6.0 also plays a role in the Application Tier's caching mechanism (but don't quote me on that).
I was curious about why Mitchell was asking about this requirement so I asked him if he could elaborate. It turns out that his company has a policy against installing IIS on systems which also function as database servers. I hadn't encountered this particular policy before, but I can understand why such a policy may exist. Mitchell was relieved to discover that he could make use of a dual-server Team Foundation Server deployment (one server for the Data Tier, one server for the Application Tier) and still meet his company's IT requirements. Mitchell agreed to let me recount his story here in case it's of any use to others in a similar situation. Thanks, Mitchell!
For more information on the system requirements for Team Foundation Server please see the Installation Guide and the Administrator's Guide.