That's odd, the Prescaler library provides the "trueMillis()" and "trueDelay()" functions because it says that they need to be rescaled according to the clock speed. Maybe I'm missing something...
I have found that using the Narcoleptic library (Google Code Archive - Long-term storage for Google Code Project Hosting.) to sleep for a second at a time, I can get the power consumption down to half what I can get through lowering the clock speed to 1MHz (~5.5mA sleeping at 16MHz vs ~11mA not sleeping at 1MHz), so I am going to try that instead. (The button debouncing was used to set up a duration before letting it run, so it's not needed most of the time.)