Casey Johnson's entry on Nobel Intent is an interesting snapshot of how the brain may handle multitasking:
Researchers found that, when single-tasking, subjects used both of their frontal lobes to manage the work. When there were two tasks and rewards in play, each half of the brain was devoted to managing each task and tracking the associated reward. We still can't literally do two things at once, but we can at least monitor both in relation to each other.
His entry is based on findings published in Science this week.