The Microsoft Exchange Team has an interesting post about the Supportability for multiple OWA/ Exchange Web Sites on Client Access Servers in Exchange Server 2007 and Exchange Server 2007 Service Pack 1.

If you read my series of articles about Shared Hosting with Exchange 2007, that may be of interest to you.

Here’s a little excerpt:

The following outlines the general supportability guidelines for Microsoft Exchange Server 2007 with and without Service Pack 1.

1. If you are using Forms-Based Authentication for your /OWA and/or legacy (/Exchange and /ExchWeb) virtual directories, Microsoft supports a single Web site per Client Access Server. Further, the /OWA and legacy virtual directories must be in the same Application Pool (AppPool).
2. If you do not use Forms-Based Authentication, you can use as many Exchange Server-related Web sites as needed for your organization. 
   However, Microsoft recommends that you use a Microsoft ISA Server 2006 server to handle Forms-Based Authentication for your various Web sites, in this circumstance.
3. If Forms-Based Authentication must be used on the Exchange Server 2007 computer *and legacy virtual directories are not used*, you can use multiple Application Pools (AppPools) for each Web site. 
   Note:  Lack of legacy virtual directories will prevent proxy to Exchange Server 2003 mailbox servers and prevent Entourage clients from synchronizing with the Exchange Server using the Exchange service. 
4. Outlook Anywhere (formerly known as RPC over HTTP) can only be set for a single Web site, per Client Access Server.  If you wish to use an approximation of a Hosted solution for Outlook Anywhere, it would have to be done using redirection per the following TechNet topic. 
   “Configure Client Access Servers”