It will be interesting to see how flat is too flat to charge via USB, and wether we can program the Reader to warn us at that point: "Hey, if you were planning on charging me via USB, now's the time, otherwise you have XXX more page turns until you need to plug me into the wall."
The basic issue is that same as I discussed over in the iLiad thread, when folks wanted to make a USB charger cable for the iLiad. USB doesn't provide much power.
It would be interesting to test the Sony Reader and see if a double headed USB cable is supported to charge it when its flat. A two headed USB cable provides twice the juice. But if Sony didn't design the unit to support that much juice being available it won't exploit it.