Can I monitor my phones temperature? Recently in the UK we've had an unexpected bout of nice weather which means wherever I leave my phone it tends to get quite warm. I know the operating range of my phone (Sony Ericsson Arc) however is there an easy way to find out how warm it is? 

It's never left in the sun, always in the shade. If I need to root it I'm open to options - currently running stock 2.3.4 Android.
You can use TempMonitor available from Google Play Store:

System temperature reading in your status bar updating at your choice interval.
TempMonitor is the oldest CPU Temperature monitoring app on the android market. This app is built to do one thing: Temperature, and it does it well.


Some features that you might like:
The temperature can be displayed on the status bar or in the APP Screen. You can also select what you want to see:


Choose from Celsius or Fahrenheit.
Choose what temperature to display (Battery, CPU).
Choose the update interval.