One common request for Exchange 2010 regards the possibilty to install one in shared hosted environment.
About this we have to consder the following aspects:
1. The hosting mode in Exchange Server 2010 needs the SP1 level. Exchange Server 2010 SP1 supports hosting deployments and provides hosting partners with the core Exchange features so that they can be deployed to multiple customers in a single installation.
2. The following resources can help you deploy hosting in Exchange Server 2010 SP1.
Deployment Checklist for Multi-Tenancy
Hosting Deployment Guide for Exchange Server 2010 SP1 (beta)