Good news all around.
There is no discernible loss of battery-life when using the 7200rpm hard drive (based on a stopwatch test doing “light” work, e.g. Java development, running our server, reading email, browsing, playing iTunes). As an added bonus, I haven’t noticed any additional heat via the left palm-rest (or via TemperatureMonitor).
The fact that the faster drive performs on par (from a heat/load perspective) with the 4200rpm drive isn’t entirely unexpected. There was a working hypothesis that because of the better bearings and identical power consumption performing read/write (tho’ it does use more power during spinup) there wouldn’t be more heat or less battery-life. Thankfully that hypothesis has proven true for me…