Vodafone's warning, which, as noted above, was reissued on Monday, said:
We’re aware of an issue caused by Apple iPhone 4S handsets that have been upgraded to iOS 6.1 which impacts performance on 3G. Some customers may occasionally experience difficulty in connecting to the network to make or receive calls or texts or to connect to the internet.It would seem that VFUK and 3 Austria are a big late to warn customers against upgrading to the new version. Apple released iOS 6.1 on Jan. 28. It added LTE support for more carriers (36 iPhone, 23 iPad) and the ability to purchase Fandango movie tickets via Siri, among other things.
Apple is working on a solution to their software issue. These connection problems are intermittent. While Apple’s investigations continue, we would recommend that anyone who has not yet installed iOS 6.1 on their iPhone 4S should delay doing so until Apple has confirmed that the problem has been fixed.
Notably, although 4G phones will back down to 3G when necessary, the iPhone 5 and the cellular versions of the iPad mini and third- and fourth-generation iPads are the only iDevices that support 4G LTE. Older devices, such as the iPhone 4S, are 3G only.
The 3G bug is only one of a number that are attributed to iOS 6.1. Other bugs that have been seen -- or at least, reported -- are battery drain issues and an excessive logging issue on Exchange. That second problem has led some IT administrators to ban iOS 6.1 from their environments.
Thus far, Apple hasn't commented on the bugs, but that isn't unheard of. An earlier report said that 6.1.1 -- whatever bugs it fixes -- is already in developer testing.
Update: Apple has pushed 6.1.1, for the 4S only, to address cellular connectivity issues.