We told everyone right away as we were just too excited to keep a secret! I agree with the m/c thing after having a SIL go through that at 8 weeks preg. It's a lot easier to get support if people (at least your family) already know that you're pregnant.
The other reason I have heard people telling right away is if their m/s is out of control. A friend of mine would have to run out of the room with no warning cause her m/s came on so quick - so she thought it was better to explain than to offend.
I think it takes us up to a day to get used to the idea and then we start picking up the phone etc... it's quite fun