Apple release iOS 6.1.1 on Monday, a rush release to fix, "an issue that could impact cellular performance and reliability for iPhone 4S." This appears to be in response to some problems with 3G networks in Europe that surfaced with iOS 6.1.
Earlier on Monday, Vodaphone in the UK and Three in Austria both warned owners of the iPhone 4S not to upgrade to iOS 6.1, which was released on January 28th.
The update is available now through iTunes or through your iPhone. Your mileage may vary, but it's being rolled out as we type.
Other than the cellular performance mentioned above, the patch notes do not mention other changes.