When watching HLV, I honestly thought that it was just a ploy of Mycroft/Johns to get Sherlock back from exile. I didn't think that Mycroft would show his 'shock' at how it was possible, I would have thought that he would be all stoic and put his big pants on to deal with it, and John hardly seemed bothered! In fact, Mycroft got over the 'Its not possible, its simply not possible!' fast enough to seem coy on the phone to Sherlock about coming back from exile, and I reckon John knew it was going to happen, from his blatant nonchalance when Mary is panicking. However, I do really hope that Moriarty is back - it wouldn't be quite as exciting without him ;)