Before discussing the porting process, first have a look at which kinds of technologies are available for the cell phone user interface.
WAP is one of these technologies, of course. It is an XML-based markup language designed especially for wireless browsing. WAP is a bit like HTML (with some differencesin the tags, for example). Most of the new cell phones have WAP browsers, so there are plenty of devices on the market. The problem is that people are not yet familiar with using WAP. And even though WAP is somewhat similar to HTML, new WAP pages (or WAP interfaces) need to be created when porting a service.
Some cell phones support HTML, XHTML, or a subset of them. They might not be able to show all the graphics and elements on the page, but the markup language is basically the same. Usually, developers are still forced to make specific interfaces for these devices because the content they can display is a lot more simple than the content a desktop computer can display.
It is also possible to develop complete applications that are designed to run in the cell phone. These applications enable the developer to design the user experience as they wish. There are many technologies: Symbian (Series 60 being the most famous), Palm OS, Microsoft Windows Mobile (previously known as Pocket PC), and MIDP (the mobile Java), to name a few. They all have their advantages; some developers are used to developing with Java and some with VB, so there's something for everyone.