The battery was shot. It wouldn't hold a charge for more than a few minutes. I was using an external battery case with it, but when that battery ran low and I didn't notice, the phone would crash. The last time it crashed, it rebooted only to the "plug into iTunes" screen, and I was left with a jailbroken, unlocked brick.
As I had never opened an iPhone before, I took my time with the repair; it took me about an hour. The parts that made me the most nervous were first, taking the screen off. You have to use quite a bit of force to remove the glass from the metal bezel and I was afraid it would break loose taking a few cables with it. The other was removing the actual battery. It's glued in well, and again I was afraid of damaging the chassis as I pried away. I didn't have a hair dryer, as is recommended to loosen the glue, so I used a gas stovetop burner - probably not recommended, but it helped!
I now have a "new" phone, jailbroken, unlocked on iOS 4.2.1 The crash actually helped me, as I added multitasking this time which I love! The Hardest time with the whole process actually was filling out this form! Below this box it says drop images here. After I had written my Pulitzer prize winning essay, I dragged an image onto that box, and it took me to another screen, and my essay was never to be seen again. So, sorry, all you get is this note now!