Nokia has published a new tool for developers, Nokia Energy Profiler to know and quantify the power consumption of their applications. Presently it is compatible(read working) with S60 3rd FP1 devices only.
This specialized tool would be helpful mainly in knowing if a particular approach is more/less power consuming. Also mostly if the Power and Resource management Guidelines are followed, then this tool would rarely be required.
Actually this tool reminds me of Batmon, a tool to know the battery parameters. It was a cool tool which worked using some great hacks. Need to find where that tool is once more ;).