As always... ***SPOILER ALERT***
Jim Moriarty is supposedly alive. Or is he? The end of "His Last Vow" is far from conclusive. The only thing seen by the characters in that scene is a picture of Moriarty. His little blip after the credits may have been an extra for TV that was thrown in specifically thrown in as a smokescreen (typical Moffat/Gatiss trickery). It does seem a little too convenient that Moriarty chooses to come back right as Sherlock is sent off to a sure death.
Sherlock could have easily had a plan in place should he need to escape an exile attempt. He very likely had an accomplice in place should the need ever arise. This accomplice would be ready to hack into the TV feed in London and broadcast that message.
The other possibility is that someone (other than Moriarty) knew what was happening to Sherlock and broadcast that message without being told by Sherlock. This person would need the ability and means to hack into the TV feed. Furthermore, they would need a good reason to keep Sherlock in London.
Who could have been that accomplice or that Good Samaritan? Mycroft seems unlikely. He seemed genuinely shocked Moriarty was back. On the other hand, when has Mycroft ever been straightforward? Molly is another interesting possibility. She could be an accomplice, but she could not have acted alone. We have no evidence she'd be clever enough to pull something like that off without Sherlock's help. A third possibility is The Woman. She had help from Sherlock to escape/fake death at the end of "A Scandal in Belgravia", and she apparently sent him a flower while he was in the hospital. Clearly, they still keep tabs on one another. The last possibility is that Sherlock himself set that plan in motion before he went Magnussen's house. He could have easily had Bill Wiggins or another member of the Baker Street Irregulars on call to put that plan in motion should things turn ugly at Appledore.
Or maybe Moriarty really is alive... Or maybe it is Moriarty's brother, James Moriarty.... Who knows? I just hope the truth is not boring.