As of 2013, 29 of the 330 (8.8%) NASA astronauts have had 0 flights. Removing the 2009 rookies, 20 of 321 (6.2%) have had 0 flights. Incidentally, none of these 20 are still astronauts.
11 of 312 (3.5%) that survived long enough to have a chance never flew.
http://www.nasa.gov/pdf/740566main_current.pdf
See, Elliot M., Jr.; Rookie year: 1962; DIED: February 28, 1966 in crash of T-38 jet
Bassett, Charles A., II; Rookie year: 1963; DIED: February 28, 1966 in crash of T-38 jet
Chaffee, Roger B.; Rookie year: 1963; DIED: January 27, 1967 in the Apollo 1 spacecraft fire
Freeman, Theodore C.; Rookie year: 1963; DIED: October 31, 1964 in a T-38 jet crash
Williams, Clifton C., Jr.; Rookie year: 1963; DIED: October 5, 1967 in T-38 jet crash
Graveline, Duane E.; Rookie year: 1965
Michel, F. Curtis; Rookie year: 1965
Bull, John S.; Rookie year: 1966
Givens, Edward G., Jr.; Rookie year: 1966; DIED: June 6, 1967 in an automobile accident
Chapman, Philip K.; Rookie year: 1967
Holmquest, Donald L.; Rookie year: 1967
Llewellyn, John A.; Rookie year: 1967
O'Leary, Brian T.; Rookie year: 1967
Smith, Michael J.; Rookie year: 1980; DIED: January 28, 1986 in the STS-51-L accident
Thorne, Stephen D.; Rookie year: 1985; DIED: May 24, 1986 in airplane crash
Cagle, Yvonne D.; Rookie year: 1996
Caldeiro, Fernando; Rookie year: 1996
Loria, Christopher J.; Rookie year: 1996
Robertson, Patricia Hilliard; Rookie year: 1998; DIED: May 24, 2001 of injuries from the crash of a private plane
Woodward, Neil W., III; Rookie year: 1998