The International Review of Research in Open and Distance Learning recently published an e-journal volume dedicated to issues in mobile learning. Here's a blurb:

"This special IRRODL issue on mobile learning is timely because of the proliferation of mobile technology in society, globalization, and the need to re-examine how learning materials are designed and delivered for the new generation of learners. In today's world, people are on the move and are demanding access to learning materials and information anytime and anywhere. At the same time, there is increasing use of mobile technology in different sectors of society to meet the needs of people on the move."

To check out their articles on PDA learning, student retention with mobile learning, and more see: Mobile Learning.