I've spent much of my recent life extending the Windows Mobile experience beyond the confines of the Pocket Office apps.    Once people get the idea that they can get their mail, contacts, calendar and task on a mobile device its amazing how this serves to get them into the thought process for other line of business applications.   Certain themes popup again and again namely:

Timesheet recording
Expense logging
Logging mileage and travel
Booking leave
Purchase order approval
Mobile CRM

I must have now done several  different versions for all of the above.    However needless to say for each audience I have created these applications they lend them self so well to be used from mobile devices.   I think getting the infrastructure in to support Outlook, to facilitate the mobile messaging side of things is a great first step.  Once done this is a great platform to extend and build top quality Mobile LOB applications.

If you don't believe me look at

http://www.Microsoft.com/casestudies/casestudy.aspx?casestudyid=53683