If you are planning to enable any of your existing application on mobile (be it for your consumer or employees), make sure that you have the needed web services is place before you start, in most cases these are the same services (assuming you have service oriented architecture in place) which are accessed by your intranet or online applications currently.
When it comes to mobile you may not need all the data your existing service returns or you may wish to call multiple services before you respond to one mobile request (to limit the number of server calls). To accomplish this consider building mobile service layer, containing wrappers around existing services and also there may be services in this layer which are specifically built for mobile clients
Other architecture considerations for building a mobile app could be building synchronization component for apps which may need offline content, Notification services to push ad-hoc or scheduled reminders, Integration with Third party applications, DRM, Mobile Analytics and Security (MDM).
Typical Enterprise Mobility Architecture