Through June 2012, Wind was 3.8% of electricity generation, Nat Gas 30.4%, Coal 35.4%, Nuclear 19.5%, Hydro 7.9%. Wind generation grew 16.3% since the first 6 months of 2011; NG grew 34%. 

Source: http://www.eia.gov/electricity/monthly/ modified to combine Table 1-1 w/ 1-1a