“From when to choose mobile Web services to the overall design guidelines to the value types to use in mobile Web services, this article addresses many of the design considerations you need to ponder when developing Web services for mobile devices. Learn how to decide when to use Web services, what things to consider when you design Web services, and what to keep in mind when planning mobile Web services.”
SOAP or even any form of XML seems a bit overkill to me. If you are targeting enterprise applications with customers who don’t care much about money then ok, but if you are offering services to end-users and especially ones that live in places where GPRS/Edge is so horribly expensive (I am talking about my beloved switzerland) you’d rather use a very simple httprequest or even socket communication to avoid sending too much information which will cost money to the customer.
When you update information on screen every minute you’d rather download 200 bytes per request than 2kb or some end-user is going to faint when the bill arrives.