Upgrading EAS in an Exchange 2003 Environment to Exchange 2010: Some may’ve environments that've Internet facing AD sites and non-Internet facing AD sites. As part of upgrade process, you’ll be following a model where: Ensure all Exchange 2003 servers are SP2; Deploy Exchange 2010 CAS, Hub Transport, and Mailbox in "Internet Facing AD Site"; Have legacy Exchange servers in "Non-Internet facing AD site" (if they exist). In other words, it’d look something like this for an Exchange 2003 upgrade/co-existence:
In order to introduce Exchange 2010 into "Internet Facing AD Site" and support Exchange 2003 mailboxes, move primary EAS namespace associated with Exchange 2003 Front-End servers and associate it with Exchange 2010 CAS array. For more information on the detailed steps required to support coexistence process see TechNet, or Deployment Assistant.
Full Article: Upgrading Exchange ActiveSync to Exchange 2010