Apple recently launched the MacBook Pro SMC Firmware Update 1.3, an update that corrects the behavior of 15 and 17 inch MacBook Pro fans while working under heavy load conditions.
The update, once downloaded, can be found in the / Applications / Utilities folder, but will run automatically. The user must follow the instructions in the update application to complete the process.
More information can be obtained in this Apple support article. SMC Firmware Update 1.3 requires Mac OS X 10.4.11 Tiger (or higher) or 10.5.7 Leopard (or higher) and weighs just 833KB.