There are a whole bunch of articles that note that IPK looks like he is done for the year. Here's one from the Daily News:

Ian Kennedy, who was scratched from his Saturday start with back spasms, is unlikely to pitch again during the regular season because of the injury. "It's nothing more than it was, but it's working its way out of his system," Torre said. "It's a little discomfort. I'm not sure he'd be able to throw enough to be in a situation where he could (pitch)."

The articles don't definitively say whether or not he'll pitch in the playoffs but Torre's quote is pretty telling. Here's Newsday's version :

Ian Kennedy's impressive season may be over. Torre said the rookie righthander, who started 2007 in A ball, is still troubled by a back strain and may not pitch again in the regular season. That would probably nix him from a postseason roster spot.

and the NY Post:

One pitcher who probably doesn't figure into the Yankees' plans for the remainder of the week and perhaps into the playoffs is Kennedy, who missed his last start with soreness in his upper back.