The iPhone and iPad are sometimes criticised for not having removable battery packs. But the trend (as with the latest Airs and MacBook Pros) is inexorably away from swappable battery packs to built-in power. I’ve never had a problem with my iPhones, accepting that I need to charge them every night. But I always carry a rechargeable battery pack in case I do run out of juice during the day. These packs (I am currently using a HyperMac Micro) are light and easy to carry around.
For the ultimate protection, though, you need the Little Black Box which takes four ordinary AA batteries (or rechargeable AA batteries if you wish). At just under $30, it’s a neat solution for travellers and I’m putting one on my want list.