Immediate gratification is characterized by higher emotional (limbic) system activation, and decisions are made when the emotional brain battles with more analytical areas for control. Interestingly, emotional brain is not just a 'nuisance' sabotaging control by the analytical brain. The sequence of brain activation following emotional activation suggests that emotions activate working memory areas (motivation!) to help the work get done.
Pathways in Human Brain, October 15, 2004 Press Release - National Institutes of Health (NIH)
The neural system that bridges reward and cognition in humans: An fMRI study -- Pochon et al. 99 (8): 5669 -- Proceedings of the National Academy of Sciences