In cases where this happens, it is most likely the result of the SMC update not being applied properly, as reported by this Apple KB article. If you are experiencing high fan speeds after installing the iMac SMC firmware update 1.0 on your late-2009 27-inch iMac, follow these steps:
- Check to be sure you have the proper SMC firmware. Click on the Apple icon in the menu bar and select About This Mac.
- Click More Info... at the bottom of the window.
- In the Hardware Overview pane, check the SMC Version (system) field. If the field displays 0.00, the SMC update was not applied properly.
- Reinstall the iMac SMC firmware update 1.0. You can download it here.
- Should reapplying the update not solve the issue, you should take your machine to an authorized AppleCare technician, your local Apple Store, or call AppleCare at 1-800-APL-CARE.
To read more about issues surrounding the 27-inch iMacs, check out this article from late last year, just after they were released.