Overclocking isn't worth the cost to battery life, I've underclocked mine to 1Ghz, I can't notice any difference but it ekes a bit more life out of the cells.
1) i have a clock adjuster with ONDEMAND 'governor' - 200MHz-1200MHz: doesn't it mean i am saving battery? ie when at rest it sits at 200MHz? overnight it barely used any battery when i slept.
following from that, if i made it 200MHz-1500MHz, i am sure the battery consumption would be very similar (i currently don't do heavy processing like 1080p video, gaming etc but would like to OC "just because I can")
2) vodafone australia
hmm ... i'll ponder the OC kernel(s) when i have time and energy
That is standard behaviour. The ondemand governer will ramp CPU speed up & down depending on demand for processing power. WHen the phone is idle it will spend most of its time sitting at 200Mhz. Background applications will often cause CPU spikes, particularly with the "Android OS" usage issue present in 2.3.3.
Overclocking to 1.5Ghz will cause CPU spikes to run at 1.5Ghz instead of capping them at 1.2Ghz. It needs to be sustained for more than a couple of seconds for it to ramp up that high though. Depending on what's running in the background it may make very little difference. Generally speaking most media playback etc won't cause the CPU to run at full speed continuously. Even at 1.5Ghz, the display would still be using most of the power assuming that's running a fair bit.
The Super AMOLED screen will use less power the darker the image it's displaying is, (or even the closer to the red end of the spectrum I believe), so a black carbon-fibre desktop background will use less power than a bright sunny sky background.
Generally speaking, the less apps you have polling in the background while idle, the less effect an overclock will have.