Upgrading to exchange 2010 from exchange 2003 or exchange 2007 or both in a mixed org is a huge topic. You can start by reading TechNet and working your way through the various upgrade scenarios OR you could navigate to the newly released Exchange Server 2010 Deployment Assistant.
The Exchange Server 2010 deployment assistant is working for the 2003 upgrade scenario only at this stage, but already it's quite amazing – why do I say that? Once you've answered the questions you are presented with a HUGE amount of usable information, with “how do I do this?” and “How do I know this worked” sections in the documentation.
The wizard moves from pre-requisite checks through to specific configuration on deploying not only specific roles, but specific configuration items like how to make you cas internet facing, though to moving public folder data from Exchange 2003 to Exchange 2010 are specified in enough detail to get a rolling start on not only planning but the specifics on how to do the actual installation and migration over to the newest version of Exchange.
It would be nice to see some more configuration specific questions like “Are you planning to use High Availability features in Exchange 2010” which would lead to DAG configuration items, etc, etc. But that wish list could grow massively and keep someone busy for the lifetime of the product…..
In my estimation this is a fantastic tool!