Further to the above (experimental) script, I've just taken it out on the road for a few hours, to see whether I've done the overall performance any good. Results are mixed... the good news is that I haven't produced a single crash and that the impact on battery life (whilst not yet measured) is (subjectively) acceptable.

The impact on performance/responsiveness is the mixed bag; once the overclocking happens, I can really tell the difference between Opera running at 1100 and Opera running at 805 or 900. However, it can take a couple of seconds to surpass the up_threshold and increase the clock speed, so it shows the benefit more when it's rendering the first web pages loaded than while bringing up Opera itself. I could solve that by temporarily locking a frequency, but that would guarantee to waste more power on every unlock, whether it turns out to be needed or not.

From watching conky while playing with commonly used applications, I've seen quite a lot of the CPU hitting 50-60% usage (at 500MHz) for a while before it triggering the desired speed boost. OK, the system may not be CPU-bound at that point, but two or three seconds later we hit the up_threshold. This makes me tempted either to lower the up_threshold and make our hair trigger even finer, or to keep it at 75 but lower the minimum speed to 250MHz. My intention is that merely unlocking the phone (such as to check the time) should not do any more than it has to (staying at minimum speed would be nice), but that unlocking the phone and e.g. starting Opera should trigger 1100MHz before I ask it even to give me the start page.

Again, I don't warrant that this is either clever or desirable, but I'm having fun. Comments or refinements welcomed...