With the introduction of 3G networks, multi-tasking becomes more relevant. A user could download a music track while navigating information about the band on a portal site. An IM session could run concurrently with a multi-player game, allowing the participants to chat to each other while the game unfolded.
However, this does assume an evolution in the way people use their mobile devices. Back in the early days of the Palm platform (around 1998 I think), Palm conducted some interesting research among its users. It highlighted the tendency for users in the mobile environment to interact with their devices multiple times throughout the day, but for very short periods. In contrast, a fixed user would typically spend a few lengthy sessions immersed in their computing environment. Multi-tasking is an inherently immersive activity, whereas by necessity the mobile environment is a casual medium - it never fully captures the user's attention because there are many other distractions around them.