The following are the link of updates of iPhone OS Firmware version 3.1.2.
The update automatic makes Facebook for iPhone update that got improvements such as error/Crashes, incorrect match on Address Book synchronization, and Incorrect match error.
Here are the download link:
Download iPhone OS 3.1.2 for iPhone 2G
Download iPhone OS 3.1.2 for iPhone 3G
Download iPhone OS 3.1.2 for iPhone 3GS
tag: iPhone OS Firmware 3.1.2 Update, iPhone OS Firmware 3.1.2 Download