..same here. Like April, I'm pretty convinced this has less to do with ePub and more with not properly suspending the processor when in sleep mode.
If you're having this problem after the upgrade, please let Sony know. Perhaps if they are convinced it's not an isolated problem, they'll figure out the software fix.
If anyone is curious, here's some current measurements made by DiabloNL - back when we were complaining about the battery going flat in a month