Thanks to Brett who pointed out that we have now released the Data Protection Manager (DPM) 2007 Storage Requirements calculator to the web.

As he says:

http://blogs.technet.com/dpm/archive/2007/10/31/data-protection-manager-2007-storage-calculator.aspx

This iteration of the calculator only focuses on storage planning for Exchange Server 2003 and Exchange 2007, but in the future can be updated to include other technologies like SQL and Sharepoint. 

For Exchange 2007 planning, it does leverage the Exchange 2007 Mailbox Storage Requirements Calculator (http://go.microsoft.com/fwlink/?linkid=84202).