I think they're supposed to be using ActiveSync, which they licensed from MS to add Exchange support to the iPhone 2.0 software. It supports push and works well on the iPhone, and it's the same thing MS uses for the Outlook Mobile client in Windows Mobile. It won't have every feature of Outlook on Windows but I expect the core functionality (which is all 90% of the users make use of anyway) to work well. If I really need Outlook I run it in Parallels using coherence mode and it's almost like a Mac app. Well, not really.
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