Linda Stone explores the difference between multitasking and continuous partial attention (or CPA).

When we multi-task, we are motivated by a desire to be more productive and more efficient. Each activity has the same priority [...]. With multi-tasking, one or more activities is somewhat automatic [...]. That activity can be paired with another activity that's automatic or with an activity that requires more cognition, like writing an email or talking on the phone. At the core of multi-tasking is a desire to be more productive.
In the case of continuous partial attention, we're motivated by a desire not to miss anything. There's a kind of vigilance that is not characteristic of multi-tasking. With cpa, we feel most alive when we're connected, plugged in and in the know. We constantly SCAN for opportunities - activities or people - in any given moment. With every opportunity we ask, "What can I gain here?"

An extremely relevant observation, considering the amount of functions that mobile devices bring to our pocket. It used to be that you could not watch TV during dinner in a restaurant because there simply wasn't a TV present. You didn't have to make the choice. Now, simply enjoying your dinner, is choosing or not to watch a movie, communicate with people that are not at the table, manage your calendar, or look something up in wikipedia. Not that I'm worried. Just slightly puzzled.

See also:
> Linda Stone's website on Continuous Partial Attention
> Multitasking and CPA in Wikipedia
> Nokia Misrepresents Multitasking Survey Results