Fundamental misunderstanding of telecoms
There seems to be some fundamental misunderstandings out there about how cellular comms actually work. The encryption keys which have been stolen were only ever used for the wireless link - i.e. between phone and cell tower, and even then could be circumvented via active attacks. Within the carrier networks the calls are in the clear.
Some carriers may encrypt some parts of their network - there is no guarantee though, and the carrier wouldn't necessarily tell you either way. So in short your comms were _already_vulnerable_ if you relied on this encryption. That's why you use multiple layers of crypto, and use end-to-end encryption. Yes, having the keys makes passive interception and decryption easier, but if you were relying on it for your security then you were an idiot.
In the UK, these keys are in theory not much use anyway. That's because the exact same warrant would be required to use these keys within the UK as would be needed to order the carrier to do cell interception - in fact, this was confirmed in the recent trial which found that GCHQ had been breaking the law because they hadn't made this fact public. The keys could be misused, yes, but oversight in the UK is actually pretty good - again, the recent "GCHQ broke the law" was a pretty technical finding, not intimating willy-nilly interception of anyone they want.
The US is a different situation - the yanks have bugger all oversight over their intelligence services. They need to sort their act out, but I expect to see pigs flying first.
Where the keys are useful is intelligence in unfriendly locations, where the UK government cannot ask the carrier to do intercept direct, and cannot ask the local intelligence/police to help as they're not trusted. For example, it would be dangerous to ask the ISI to do cell intercept on the Pakistani Taliban, due to supporters within the ISI itself who would leak the fact that specific phones were being targeted.
All countries spy on each other. The only countries that don't, only don't because they cannot afford to.
The more iffy area is spying on employees of Gemalto and carriers, in order to gain access. The leaked documents highlight that specific only K, Ki, and IMSI info was being searched for. There was no mention of looking for personal information for blackmail etc, and furthermore these were all work email accounts - I didn't see any evidence of searching home accounts for embarrassing details etc.