Exchange Pre-Deployment Analyzer - nice for catching the gotchas before they getcha

The Exchange Pre-Deployment Analyzer performs an overall topology readiness scan of your environment and provides you with a list of decisions that need to be made before you deploy Exchange Server 2010.

The checks performed by ExPDA are similar to the pre-requisite checks implemented (via Exchange Best Practices Analyzer) in the Exchange 2010 Setup program; in fact ExPDA is based off the Exchange Best Practices Analyzer (ExBPA) engine. However, unlike Exchange 2010 setup, this tool focuses only on overall topology readiness and not the ability to run Exchange 2010 on the local computer. The scan also performs a deep analysis of each existing Exchange 2003/2007 server to verify that it has the necessary updates and configuration in-place to support Exchange 2010.

Another good thing to note that hasn't been documented very well, but I have seen happen a few times already:

If you plan on ever deploying Exchange 2007 portions with Exchange 2010, for instance if you are coming from an Exchange 2003 environment and need a few Exchange 2007 servers due to 3rd party applications not catching up to Exchange 2010 yet, make sure you run the schema update for Exchange 2007 before running the schema update for Exchange 2010.

If you run the Exchange 2010 schema update without ever having any Exchange 2007 schema in place, you can no longer install Exchange 2007. You can go back to AD backups and recover, and I have heard of some unsupported/daring changes to schema versions to force it to work, but for pre-deploying, make sure you take this into account.

A common scenario like this is with Cisco Unity or some archival products. Little by little, they are catching up to Exchange 2010 with support.