Nice copy and paste from here: http://policelink.monster.com/topics/74818-40-cal-or-9mm/posts
One must remember that when the .40cal S&W round was developed (and its 10mm cousin) the FBI was looking for a duty pistol that provided the "knockdown power" of the .45cal with the round capacity of the 9mm. It was at the same time when the CHP (California Highway Patrol) was also looking to make changes with its standard issued duty weapon. At that time, the FBI selected the 10mm. The CHP selected the .40cal S&W, citing the fact that it had similar "knockdown qualities" of the .45cal while maintaining the round capacity and controllability of the 9mm.
As a direct result of the North Hollywood Bank of America robbery and subsequent shootout that followed, the LAPD conducted a review of its standard issue equipment which included the 9mm Beretta 92fs and the Ithaca model 37 12ga pump / Remington 870 12ga pump shotguns. Comments that came directly from the officers involved as they engaged two suspects heavily armed with AK47 fully auto-matic assault rifles and completely covered in Kevlar included that nothing the LAPD line officer had was able to provide the "knockdown power" to stop them. They watched as shot after shot of 9mm and .00 buck went into the 10 ring and bounced off of the suspects barely causing them to flinch. All commented that at a minimum, a .45cal pistol round would have been more effective as would rifled slugs in the shotgun. Neither of those were options were available and officers in the LAPD were prohibited by policy to use them. Granted, the LAPD North Hollywood Bank of America robbery was unique, when considering the method(s) and the equipment of the suspects and that not everyone we might engage will be wearing Kevlar from head to toe. I also know that the officer's acquisition of several AR-15's from a local neighborhood gun store was what ultimately turned the tide in the officer's favor long before the arrival of SWAT officers. Still, the comments simply could not be ignored. BTW, similar comments have been made about the 9mm and its ineffectiveness as officers have engaged suspects high on PCP. When the results of the LAPD review were completed, policy was amended to allow officers who choose to qualify and carry pistols in .40cal S&W and .45cal to do so. Policy also was changed to include rifled slugs as part of the issued shotgun ammunition. The LAPD has also introduced AR-15's into many of the marked patrol units making them more readily available.
In the end, whether it is mythical or not, there is much to be said about "Knockdown Power" or officers would not be choosing the .40cal S&W and the .45cal over the 9mm if they had the choice.