Knowing that I would be on a long international flight & back recently, I bought a HyperDrive external battery to take with me. Of course, it somewhat destroys the idea of travelling light, but I wanted the additional access to power even while in the air [we were in coach, where there are no plugs in the seats].
I bought the MBP-150 HyperMac External Battery for the MacBook [Air, which I have] - it comes with the type of plug for the Air plus you can not only use it to power up your computer but also your iPod, Touch or iPhone. It added about 5 hours of time to my computer, although it may have been longer since I didn't use up all the juice and don't really know.
In any event, if it makes sense to take an extra battery with you to power up your Mac and iPod/Phone etc., this seemed to work quite well.
