MSExchange Team released a great tool to help mail administrators to move from their current environment to Exchange Server 2010. The name of this web interface tool is Exchange Deployment Assistant and it can be found at http://technet.microsoft.com/en-ca/exdeploy2010/default.aspx#Home.
At this point the only option available is Upgrade from Exchange Server 2003 and the other transition scenarios will be covered soon.
You will be asked a couple of questions about your environment and based on your answers you will have a list of steps to be performed and those tasks are explained step-by-step which makes really easy a migration procedure.
You can also send a lind of the current step by e-mail clicking on Send or download the checklist (Download Checklist link).