With a few weeks to go in the season, it's down to 3 pitchers in the NL: Ubaldo Jimenez, Adam Wainwright and Roy Halladay. Other pitchers in the NL are having great seasons, such as Mat Latos and Tim Hudson, but there's not enough time for them to catch up.
NL - Wins
18 W: Ubaldo Jimenez, Adam Wainwright, Roy Halladay
15 W: Tim Hudson, Chris Carpenter, Bronson Arroyo
If the pitchers with 18 had high ERAs or one of the pitchers with 15 wins had 1 or 0 losses or a historically low ERA, that would be different. As it is, all three of the pitchers with 18 wins have low ERAs (between 2.44 and 2.75), and none of the pitchers with 15 are having incredible seasons.
Ubaldo Jimenez: 18-6, 2.75
Roy Halladay: 18-10, 2.44
Adam Wainwright: 18-11, 2.50
There are many stats that could be examined in deciding a Cy Young Award, but here is something to consider: Since 1995, out of 30 possible awards, only 1 (!) winner had over 9 losses: Pat Hentgen, who won it playing with the Blue Jays in 1996 after going 20-10. In fact, only one other pitcher had more than 8: Randy Johnson (17-9) in 1999 for the D-Backs, but he had 364 SO.
Yes, it happened 5 times between 1991 and 1993, but that was a long time ago, and the last 15 years are probably more telling. That's not to say that Roy Halladay shouldn't win it because he'll have at least 10 losses, but any pitcher trying to win the award with that many losses is swimming upstream, and, obviously, the same goes for Adam Wainwright.
Halladay is going to beat Jimenez on a number of fronts (WHIP, innings pitched, strikeouts, ERA, SO/BB. Jimenez could get credit for pitching so well as a member of the Rockies, which has not happened very often.
Prior to 2010, the all-time record for most wins by a Rockies pitcher in a single season was 17, held by Pedro Astacio (1999), Kevin Ritz (1996) and Jeff Francis (2007). Francis was the only one with an ERA under 5.00 and it was still 4.22.
Fair or not, Jimenez is probably in the driver's seat. He has at least 3 more starts, and he could easily finish 20-6 with an ERA around 2.65. It could be very difficult for Halladay to win the award if he's 19-11 and Jimenez is 20-6 if their ERAs are similar.
That said, one bad outing could destroy Jimenez's chance if his ERA goes way up and he had to take another loss. If Jimenez slips, Halladay is next in line, and he'll have a strong case. He leads the NL in IP and he has 201 SO vs. only 28 BB.
The Cardinals are playing the Rockies at the end of the season, and while it doesn't look like Wainwright and Jimenez will face off, it's possible. Any of these 3 pitchers can win it, and none of them can afford a misstep.