Have a look at the high-level architecture of a typical system and see how different user interfaces are taken into account in a properly designed architecture.
Figure 1 A typical layered architecture.
The architecture shown in Figure 1 is a layered architecture, and each layer handles different kinds of operations. For example, the Database layer is responsible for storing and retrieving all information to and from the database. There is also a layer called Presentation logic, which makes it possible to implement new user interfaces easily on top of it.
There are already three UIs in the system (whichever system it is), and it is quite easy to add a new UI. If the system were a monolithic system, implementing a new UI (for cell phones, for example) would not be easy. At its worst, it means that the whole system would have to be done again.
When considering whether to port an existing web site or web-based service for a cell phone, you must thoroughly evaluate the existing system. If there are easy-to-use interfaces for the application logic and the business logic, porting could be relatively easy. If the database is the closest thing you can use from the existing system, you know that you'll have to do all of the business and application logic again.