I'm using iStat Pro Dashboard widget to get live system data of my mid-2010 27" iMac.
Under temperature readings it lists Ambient. Where the sensor is approximately located?
As per the manual and take apart guide provided by Apple to Apple Certified Mac Technicians: the ambient temp sensor is attached to the CPU blower (or fan).
I found this on iFixit below the page at Step 8 you will see the temperature sensor in orange
EDIT : that's the temperature sensor from the LCD
At Step 12 they mention the sensors but don't really highlight them.