There is a few reasons you may want to do this, and this is the main reason most people do not jailbreak there phones, last time I headed up north to spend some time with my family I arived pretty early as I took off work early to head up to Hanford (I arrived at about 7AM). My sister usually gets up around 11 to noon so I snuck in her room and snagged her iPhone off her desk and jailbroke it before she woke up. She was surprised to find her Springboard with new app icons and a background behind the icons.
Just so some of her friends wouldn’t mess with it I removed installer.app after I finished so they could not install anything else on the phone. All my sister was interested in was the styled icons anyway.
We’ll this goes to show you how easy it is to restore the phone, a few weeks later her screen went out and stopped accepting any input it would display things fine but would not accept any input. before I even found out about it she had restored the phone back to default settings, Still nothing. Turns out her screen went bad she took it to the apple store and they replaced it with a new unit on the spot.
All you have to do is toss it in iTunes and select “Restore” from the main iPhone page. it will even back up your phone book, contacts, even your text messages, mail and call log. You’ll end up with a stock iPhone with no data lost, other than for any custom applications that is.
So yes technically Jailbreaking your phone will void the warranty, but after a simple restore, Apple wont know. =)