I realize there are still 12 days left in September (so comparing their September pitching to other teams from completed months is a little premature), but the Giants pitching this month has been unbelieveable. The bullpen, in particular, has been dominating, allowing only 2 ER in 39.2 IP (0.45 ERA; starters have a 1.45 ERA).
Top 2 teams, ERA, by month (2010)
White Sox 2.94
Despite their pitching, they are only 10-6 because they have the 3rd worst offense in September. They are capable of scoring runs (2nd in July with 149 runs), but it's usually a struggle. The good news is that their season will likely come down to the final weekend, with a 3 game series at home against another offensively-challenged team: San Diego. Of course, the Rockies could blow past both of them, but at least the Giants will have a say in it with a series next weekend at Coors.