The third tentpole feature of iPhone OS 4.0 Apple just announced isn’t as big as the last two, but is still quite nice: persistent location.
Basically, it’s just a setting you select under Location Services. When persistent location is on, a special icon shows up on the status bar.
The big deal here is the iPhone will save battery power by not enabling the GPS unit all the time, but by triangulating a phone’s location according to which cell towers it’s near. that’s good enough for most apps, and if you want to be more precise (like in Tom Tom or Foursquare), you just go through GPS like normal.
This doesn’t seem like such a big deal to me at first… but I can’t wait to see what developers do with the API. Things I haven’t dreamed of yet, I bet.
