Roy Halladay (16-5, 2.47, Phillies) is in contention for the NL Cy Young Award again (he won it last year by a huge margin). Ian Kennedy (18-4, 2.96, D-Backs) and Clayton Kershaw (17-5, 2.45, 212 SO, Dodgers) are the other top competitors.
Some of Kennedy's numbers (WHIP, SO), don't quite match up, but that might not matter if he ends up something like 22-4. One area where Halladay is excelling, as always, is K/BB, where he ranks #1 in the ML (7.64, 195 SO, 25 BB).
7.5 K/BB, 8.5 K/9, 2.50 ERA or lower, 150+ IP (since 1900)
1999 Pedro Martinez (8.46, 13.20, 2.07)
2000 Pedro Martinez (8.88, 11.78, 1.74)
2011 Roy Halladay (7.64, 8.74, 2.47)
There's enough time left and it's close enough for Kennedy, Kershaw or Halladay (or Cliff Lee, who would need a great September to get into it), but Halladay is having his best season, and putting up some historic numbers in the process.