From the looks of Spong and Anoop's initial tests it looks as if the PSP's battery life is under five hours.
This basically means that you will have to charge the bettery before you take it anywhere. Otherwise you are in danger of running out of battery life just when you need it most. That's unless you keep it turned off most of the time and only use it when you are either riding the bus or doing some-other not so long, but still equally inactive waiting inactivity, and need something to take your mind of the horribleness of it all.