If you have a few minutes, read this excellent column from USA Today (via TheBigLead) which covers the topic of "clutch" hitting. It examines a point of view about clutch hitting that I happen to agree with: over the long term, players do not exhibit an ability to perform consistently well in the clutch. In the short term, sure, players can be clutch (see Craig Counsell, Bucky Dent) or not clutch (see Alex Rodriguez, David Wright). I believe that a .300 hitter will hit .300 in any situation, in the long term. It's a spirited discussion, usually pitting the scouting focused types vs. statistically oriented folk.
I'll leave you with some interesting stats:
Playoffs: .309/.377/.469, 123 G, 495 AB
(These numbers suggest Jeter is the same hitter in any of these situations, more or less.)
Playoffs: .279/.361/.483, 39 G, 147 AB
(A-Rod's numbers dip in the playoffs, but not as sharply as one might expect based on the media portrayal of his struggles; keep in mind that's in just 39 G, as opposed to 123 for Jeter)
Playoffs: .216./.310/.378, 10 G, 37 AB
(The playoff numbers are awful, granted, but 10 games is just too small a sample size to label anyone clutch or not clutch. Hopefully David can prove me right with a great October 2009, by adding 10-19 games to that sample size.)
And just for fun, Reggie Jackson aka Mr. October
Playoffs: .278/.358/.527, 77 G, 281 AB
(Basically the same player in all 3 situations, with a little more power in the postseason.)
The next time you hear Mike Francesca or some other windbag screaming about how the Mets should trade Wright or Jose Reyes because they are not "clutch", simply re-read this post and remind yourself how ridiculous they sound.