The name implies that Microsoft will start hosting Exchange, but that is not the case. Microsoft will be hosting services that will tie into your existing Exchange system. Whether you have in-house Exchange or hosted, you can leverage 4 different offering to increase your mail; security, encryption, archiving, continuity, and filtering.
The big thing here is reducing hardware and administrative costs as well as uptime. Mail is (in most cases) the most critical application for a business and when mail flow stops, a large part of communications are cut off. Using a hosted service for DR and filtering can greatly reduce that downtime and in the case of the Continuity offering cut it out all together.
More info at http://www.microsoft.com/exchange/services/services.mspx