I just spotted this on Apple's very own website. This is how they got the numbers that were announced at the iPod touch announcement:
"Maximum Battery Life
iPod touch can play music up to 22 hours, the new iPod nano can play up to 24 hours, the new iPod classic can play up to 40 hours (160GB model) and the iPod shuffle can play up to 12 hours on a full charge at original capacity. Apple runs tests for battery life using songs encoded at 128-Kbps AAC and songs purchased from the iTunes Music Store (128-Kbps AAC). The songs are played continuously through a playlist without interruption, with backlight and equalizer off. For all iPods, factory default settings were used, except for iPod touch, in which case all settings were default except that Ask to Join Networks was turned off."
Sorry, here's the source: http://www.apple.com/batteries/ipods.html
"Maximum Battery Life
iPod touch can play music up to 22 hours, the new iPod nano can play up to 24 hours, the new iPod classic can play up to 40 hours (160GB model) and the iPod shuffle can play up to 12 hours on a full charge at original capacity. Apple runs tests for battery life using songs encoded at 128-Kbps AAC and songs purchased from the iTunes Music Store (128-Kbps AAC). The songs are played continuously through a playlist without interruption, with backlight and equalizer off. For all iPods, factory default settings were used, except for iPod touch, in which case all settings were default except that Ask to Join Networks was turned off."
Sorry, here's the source: http://www.apple.com/batteries/ipods.html