Web Services for Mobile Phones
Wireless carriers currently offer services that allow information to be "pushed" to your phone or accessed from your phone such as weather, stock quotes, news, traffic, and sports updates. With web services, phones now have the potential to actually consume useful services.
For voice, you can allow your clients to call a number and issue voice commands by creating a VoiceXML wrapper around your web service. A good example of this is Tellme's service that gives voice driving directions using Microsoft's MapPoint web service.
Read the full article on O'Reilly's XML here.