The .NET Magazine article Exchange Server 2003: Worth the Upgrade? mentions a new set of Exchange APIs:
Microsoft is readying a new set of Exchange APIs called the Exchange Server Objects (XSO). When XSO ships (probably a few months after Exchange Server 2003), developers will finally get a remoteable, server-safe object library, based on (and designed for) .NET managed code. You can expect to hear more about XSO in the future, and it should make integrating Exchange data into the rest of your organization much simpler.
I'd not heard of this before and I couldn't find anything about it on MSDN or via Google. An article on the Register doesn't add very much:
Titanium functionality will be exposed to third parties via the XSO managed API. XSO gives developers a single, XML-based interface to write to when developing web services. This means the Exchange calendar, diary or in box could be populated with data from an outside application without using the traditional Collaborative Data Objects (CDO) route.
Jim Bernardo, .NET enterprise server group product manager, called XSO an "order or magnitude" different to CDO because it reduces development effort and is remotable. XSO also support Microsoft's Common Language Runtime (CLR).