Mason et al. have used both thought sampling (by verbal report) and brain imaging to demonstrate that mind-wandering (i.e. day dreaming that deflects conscious thought from the 'task at hand') is associated with activity in a default network of cortical regions that are active when the brain is "at rest." Individuals' reports of the tendency of their minds to wander are correlated with activity in this network.
The figures of the paper illustrate a network of many brain regions that are relatively more active in the stimulus-independent thought that occurs more frequently during practiced compared with novel tasks (verbal and visuospatial working-memory tasks were used).
There is no speculation on what the role the particular brain regions listed might be during stimulus-independt thought (SIT). I thought the last paragraph of the paper was interesting:
"The purpose of the current inquiry was to explore how and when the mind generates SIT. A more intractable question, however, is why these thoughts emerge at all. What is the functional significance of a system that wanders from its current goals? One possibility is that SIT enables individuals to maintain an optimal level of arousal, thereby facilitating performance on mundane tasks. A second possibility is that SIT—as a kind of spontaneous mental time travel—lends a sense of coherence to one's past, present, and future experiences. Finally, the mind may generate SIT not to attain some extrinsic goal (e.g., staying alert) but simply because it evolved a general ability to divide attention and to manage concurrent mental tasks. Although the thoughts the mind produces when wandering are at times useful, such instances do not prove that the mind wanders because these thoughts are adaptive; on the contrary the mind may wander simply because it can."