The Berkeley economist David Romer wrote a paper about the merits of going for it on 4th down, available here for you nerds:
http://elsa.berkeley.edu/users/dromer/papers/nber9024.pdf
The conclusion is that coaches very often do not make the rational decision on 4th down. (That is, rational in the sense of win-probability maximizing.) One of the striking results was that it made sense to go for it on 4th and goal within the 5. This is because it is (on average) worth points just to pin your opponent that deep in their own territory. It's not just that you get more points for the TD, if you don't make it then you've still pinned them within their five. Using all of the data from the NFL, starting a drive at your 1 yard line is worth about -1.5 points to you (See page 38 of paper for graph of field position values). So, even if you assume a 100% success rate on field goals, you don't need a huge probability of converting the 4th down for it to be worth it to go for it on an expected value basis.
Romer used only data from the NFL in the first quarter (to eliminate end-of-game/half issues) to make the field position value graph and conversion probabilities. The results may vary for college football.
The larger question is: what are the coaches' true goal? Maximizing win-probability, or keeping their job? Obviously, winning is good for the latter goal also, but it looks very bad when you make an aggressive play that doesn't work, even if it was the logical choice. Athletic directors likely haven't read the Romer paper either.
So, should Tressel have gone for it on that 4th down against USC? (Left as exercise for the reader.)