HP has released their sizing tool for Exchange 2010. This successor to the Exchange 2007 Sizing tool does recommendations for deployment and sizing of servers and storage and includes a (HP) bill of materials. The tool supports multi-site deployments, Database Availability Groups (DAG), DAS or or SAN-based storage and high availability and client options.
As with the 2007 version it uses a combination of technical and business requirements and projects best practices for high available Exchange configurations (when applicable). The tool can update itself and the product information it uses.
You can download the HP Sizer for Exchange Server 2010 here.