Chances are, if you have 2010 exchange server in your environment, the schema has already been prepped to accept a 2007 server.... Even migrations between 2003 and 2007 is relatively seamless ..
But the installation wizard will do a cursory check of the Active Directory to ensure that all is in place for it to be installed.
But why would you want a 2007 server? I'm curious. You'd have the same migration issues between 2003 and 2010 as you would with 2007. Mailboxes would still have to be offline during the move from 2003 to 2007 ..
Something to consider.
AlRo
System Administrator
Ottawa, Canada.