While I might comment the battery replacement issue is covered all too well, I think some might want that slip on battery pack. Here's one from Amazon for the iPhone 4 -> http://www.amazon.com/EXOGEAR-exolife-iPhone-Battery-Verizon/dp/B003WJR1H6
There are bigger (more hour) units available.
I finally got an iPhone. I really love it except for several things. I am unable to view many videos as it doesn't allow for Flash Player or Java. I think it's a licensing dispute. It also won't download Firefox. As a result, I'm unable to get many of my favorite sites. Also, the download speed is kind of slow and I dread the day when the battery has to be replaced. I heard that everything in memory will be lost; including all my phone numbers, my favorite videos, etc. I was told to just to send all the info to my PC. Finally, the battery seems to lose power very quickly; especially when viewing You Tube. I find myself having to recharge several times a day.