Apple has released an updated version iOS 5.1.1, build 9B208
, for the iPhone 4 GSM/AT&T model. There doesn't appear to be any information from Apple on what triggered the new build, though it's possible some very specific bug or glitch was identified and fixed. If it was incredibly minor, Apple would likely just have filed it away for next time. If it was incredibly major, Apple would likely have pushed it out as new update to ensure every user got it.
As it is, if you've already updated to iOS 5.1.1 you won't see the new version popup as an over-the-air (OTA) update, or as available in iTunes. If you haven't already updated, you'll be offered this new version when you do. To force an update from the previous iOS 5.1.1 build to this one, you'll have to restore to it in iTunes. (And really, there's almost certainly no reason to do that.)
iOS 5.1.1 was originally released on Monday, May 7. As of May 15, almost 74% of iMore readers had already updated to iOS 5.1.1. Today, one of the major reasons many had waited, an untethered jailbreak for iOS 5.1.1, was released.