“In response to customer feedback, the Microsoft Online Services engineering team is developing the Outlook 2003 Connector. This is an Outlook add-in that will enable customers to use Microsoft Office Outlook 2003 to access all the features of Exchange Online. This includes free/busy information when setting up meetings, and the ability to access your organization’s global address list (GAL) from Outlook when you're offline. The add-in is currently in private beta testing with selected MVPs, Partners, and customers, and we’re ready to broaden the audience.”

More here:

http://blogs.technet.com/msonline/archive/2009/09/24/do-you-want-to-use-outlook-2003-with-exchange-online.aspx