I would recommend against announcing it, without clearance from management, as you could be unaware of other things that are going on. An example, it could be that you are actually being hired to replace someone who is being made redundant, or something along those lines. You announcing it could actually put you in hot water if it reveals too much information to the wrong parties.
If you want to announce it, I would recommend clearing it with management first, or only doing so with very close friends that are aware they shouldn't be discussing these changes with other people until an official statement is made.
Apart from that, the rest depends on culture and you should refer to the other answers about that as they cover it all really well.