Next to the Indigo demo, the biggest "wow" moment of Tech-Ed 2005 for me was the case study from the Microsoft IT department about how they deployed smartphones to 60,000 employees worldwide. There were two things about that deployment that impressed me:
1.) First, when Microsoft moved to Exchange 2003, they consolidated from 74 different Exchange sites down to 5. Plus, they drastically reduced the number of Exchange servers they needed to run the infrastructure because of the topology improvements in Exchange 2003, which make it unnecessary to run a bunch of different servers (one dedicated to the mail store, one dedicated to public folders, another dedicated to OWA, etc.). Of course, the servers that Microsoft is running Exchange on now are also a lot bigger and more powerful than the servers they were running before. Still, this type of simplified infrastructure is generally much easier to manage for IT.
2.) With that consolidated infrastructure, Microsoft IT basically has only two front-end servers at each of the five locations. This not only handles front end access for mail, but it can also handle access from Windows Mobile devices such as smartphones. And the surprising thing that Microsoft discovered is that access from Windows Mobile phones adds negligible overhead to the Exchange servers. Thus, Microsoft deployed 60,000 Windows Mobile smartphones to its employees throughout the world (many of them were the super-cool Audiovox 5600).
The main takeaway is that smartphones powered by Windows Mobile are simpler to deploy, much more scalable, and potentially much cheaper (since you don't need additional dedicated hardware) than similar solutions from Blackberry and Goodlink, which currently lead the market. One caveat for Windows Mobile is that Gartner analysts have called into question the enterprise-level security of Microsoft's solution.
For more info, see Mobile Access Using Exchange Server 2003.