One thought I'd like to add to this discussion is that there is a trend among hunters and social animals to develop intelligence. Granted this is a rough generalization.
However, primates are clearly social animals and so it is reasonable to assume our ancestors lived in groups. Now some aniimals in groups aren't too bright (a school of fish for instance) but others, especially the hunters tend to be on the upper end of intelligence (e.g. dogs, wolves). Clearly the development of greater intellegence leads to more sophisticated social interactions, including the development of language. Such advances make for better hunting skills, group protection etc. My guess is that these advantages would provide the selective pressure for greater intellegence.
There are costs off-course to a bigger brain. First of all, it eats a lot of energy, so a bigger brain has to let you get aditional food to feed it. The other problem, that still exists to this date, is that getting that big head out during birth is problematic. Many women have died in childbirth and many still continue to die giving birth (clearly we're a work in progress, but our brains have got some of us out of the pickle again by giving us surgeons with anesthesia). One adaptation to this problem is to give birth the less deeloped babies (with smaller heads), but again that means more parental care ex utero, which requires our brains to give us the social skills required to pull it all off.
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