Apple updated its recently-released iOS 5.1.1 edition today for one specific iPhone model without changing the version number, likely representing only a minor bugfix in the new release. The update, which is distinguished by its build number (9B208 instead of the 9B206 built that was distributed), did not come with any release notes. For those already on the original build of 5.1.1, the update can only be downloaded through iTunes or by direct download.
The update is only aimed at the GSM version of the iPhone 4 (otherwise known as AT&T-based iPhones in the US), and not the 4S or any other models. The timing of the update, coming one day after an untethered jailbreak for most iOS devices was released, appears to be coincidental and not designed to block the exploit that makes the jailbreaking possible.