+1 for Locale. Mine automagically sets my phone to vibrate only when I enter a nearby epidemiology dept, as they tut if your phone goes off.
It can also turn your phone into a cheapy SPOT tracker thingy. For audax, I have mine set to email my wife my location on the hour + SMS in response to SMS + email in response to the presence of a mobile network if it hasn´t sent my location in the last hour. Obv, it can hook up with twitter / facebook like this, or do it directly. A cheap way of letting people know where you are.
Tasker can apparently do all the same things, use the same plugins, and do more. I suspect it would be just the ticket for someone who can code.