At TechEd 2010 in New Orleans a week or two ago, I gave a talk entitled “Using Microsoft Exchange Server 2010 to Achieve Rich Coexistence with Exchange Online”. One of the cool things about TechEd talks is that now they’re recording them and posting them publicly for access by anyone, so if this is a topic that interests you, you might want to have a look at: http://www.msteched.com/2010/NorthAmerica/UNC309
What is “Rich Coexistence”? Well, it’s when you’ve configured Exchange to run on your premises and you’ve configured coexistence with the cloud… *AND* you’ve put at least one Exchange 2010 SP1 CAS/HUB server role in your on-prem environment as a “coexistence gateway”. Once you’ve got this set-up, your Exchange organization can be configured to appear to span both on-prem and the cloud in a pretty seamless way. The talk goes into more details on what you get if you set this up, but suffice to say it’s a great, high-fidelity coexistence scenario!