The field of application and standards development for wireless devices is changing rapidly. Here are some resources you might consult in researching and planning a wireless application:
http://www.wapforum.com
The WAP Forum
http://www.w3.org/Mobile
The World Wide Web Consortium mobile access activity
http://www.egroups.com/group/wmlprogramming
The Web site for the WML Programming mailing list, including a links section with links to a wide range of WAP portals, search engines, and more.
http://www.webcab.de/index.wml
An example of a useful and broad selection of WAP services, including WAP to e-mail.