High Idle should kick in when the outside air is below 32F and engine coolant below 140F. Mine has been kicking in since the temps here have gotten around 29F. It was wierd though, one morning at 23F it did not high idle for some reason. Then the following days, same conditions, it has been high idling within 30 seconds of start up.
Some have posted that the outside air temp is sensed by the battery temp sensor. The IAT was thought to not be a good one to use since the grid heaters would kick the temps over 32F as part of normal operation. On mine, when the high idle kicks in, one of the grid heaters stays on solid for a while when it's in high idle mode. Makes me believe it is using a different sensor.