~Disclaimer~ I know wins and ERA aren't the best way to judge a pitchers season (or month) but the voters don't. Wins, ERA, and strikeouts are the equivalent of batting average, home runs, and rbi's - it's all some voters look at, whether they admit it or not.
Month by month:
Mar/April: 9-4 1.99
May 7-9 3.85
June 7-11 5.25
July 5-15 5.05
Aug 8-8 3.81
Sept/Oct 11-6 2.80
We see that he starts strong and finishes strong. The ERA is nice but it's wins that matter (or all that matters according to voters and BobMcClure) and he is not lacking in that department in his career. Let's take a look at year by year Septembers compared to his final season numbers. We can see that he has finished strong every season in his career.
2004 2-2 2.45 (8-11 3.97)
2005 2-1 3.82 (5-17 5.80)
2006 1-0 4.26 (Only 6.1 IP)
2007 2-2 2.33 (7-7 3.69)
2008 4-1 2.18 (13-10 3.47)
The last two Septembers look an awful lot like his 2009 season so is there any doubt he can keep it up? I think he can but again it's the wins that matter, he probably needs to win at least three more times and hope nobody else wins the magical 20 that seems to sway voters so much. I think 17 wins guarantee him the award even if there is a 20 game winner so a repeat of last year and he's a lock.
Let's look at some of the contenders and see how they've fared over the seasons final month.
Verlander 15-7 3.38
He leads the league in wins and strikeouts but has gone just 7-6 4.73 in September. He's probably a longshot to win 20 so he will have to hope Greinke doesn't win 3 more times.
CC Sabathia 15-7 3.56
Tied with Verlander in wins and leads AL in innings pitched. He has fared really well in September going 21-11 2.77. But like Verlander he must win 20 or hope Greinke falters.
Felix Hernandez 13-5 2.77
The only legit contender in my opinion as he is second to Greinke in ERA+. But he is in the same boat win-wise as Greinke and he's not likely to win enough to merit serious consideration. September isn't his best month either as evidenced by his 7-7 3.88 career mark.
Roy Halladay 13-8 3.13
Doc was an early season contender but has stumbled a bit lately (3-5 3.63 since the break). The Toronto contingent viewd him and not Greinke as the front runner for most of the season (of course they thought he should have won the CYA over Lee last season, some crazy stuff goes on up north). He traditionally is very good in September going 19-9 2.51. Like Hernandez he has to hope Greinke goes winless and nobody wins 19 or 20.
I don't consider Beckett, Feldman, or Weavers contenders due to their ERA's being 3.70+. It looks to me like 18 wins is going to lead the league and if that happens and Greinke ends up leading the lead in ERA, ERA+, CG's, shutouts, and WHIP while finishing second in innings and strikeouts well its very possible he could win the award with just 14 or 15 wins.