I was asked this by a school district in Washington State which had Exchange 2003/2007 already deployed.
Here are a few ideas I came up with for them:
Clean up Exchange, Active Directory, and DNS
E.g. External CAS URL of mail.school.edu defined as external IP within external DNS Internal CAS URL of mail.school.edu for internal IP and internal DNS
E.g. External CAS URL of mail.school.edu defined as external IP within external DNS
Internal CAS URL of mail.school.edu for internal IP and internal DNS
Deploy Exchange 2007 SP2 (just released here) if you have Exchange 2007 deployed
Arm your self with Exchange 2010 knowledge
Read and watch all the Exchange 2010 material available on Technet , etc.
Here are some great links:
Exchange 2010 product team blog
Build your lab
I just rebuilt my virtual lab with the newly released Hyper-V R2 running on Windows Server 2008 R2 on an x64 machine. Build AD, Exchange 200x and Exchange 2010. Document the steps, migrate mailboxes, etc
Grab Exchange 2010 RC here
Bring up Exchange 2010 side by side with Exchange 2007 (no in place upgrade will be supported)
Test online migration (no user outage during mailbox move provided they have Exchange 2007 SP2) – this allows for migrations during the day, etc.