Surprised this hasn't been said... Jailbreaking is all software side. No modification of hardware.
If you do a simple restore in iTunes, you will have zero trace (essentitally) of anything and can proceed to make any warranty claims, etc, through Apple. Just clearing that up for the lot of people that are so against Jailbreaking who clearly don't really know what it does.
Sure, lots of people use it to install free Apps from the App store. I myself do it occasionally to try an App to see if I like it. If I do, i'll purchase it on the Appstore. If I don't, I'll delete the App and never look back. Many other people are the same way.. why should a Developer get $$ for making an App look pretty without providing any proof of it being functional, fun, stable, etc.
There are more positives to Jailbreaking as well, most of which have been said.
MyWi is a great tethering solution, I prefer it to the Official tethering / PDANet because it actually creates a mobile hotspot, usable by other laptops, iPods, etc. Instead of tethering by USB / Bluetooth which can be tricky / time consuming to setup. (Time consuming vs just connecting to a WiFi network).
SBSettings is another great app, it makes turning down Brightness, Wifi, 3G, Bluetooth, ETC. off/on a 2 second process. Just very conveinent.
Also use an app called mAdvlock that lets me lock certain Apps with a password, such as Mail or SMS. When I give them my phone I don't want people going through my stuff - so this helps tremendously.
There are many, many, little apps and tweaks that will help customize the phone and provide functionality that Apple has been slow to bring out or will not bring out (i.e Tethering, Multitasking, SMS Text tones...). The Jailbreaking community has been living with these features for years, iOS4 is just bringing these. Sure they might be a little better written, but "IMO" nothing that substantial.
To everyone saying how many errors / crashes / etc, if you go into Cydia and install every god damn winterboard theme, make your phone look like the Matrix, have a million Widgets on your home/lock screen, well, of course you will.
If you're a controlled user that only installs what you NEED and for functionality, well, you will have a minimal amount of battery life decrease and you won't have any crashes or errors, if you know what you're doing.
Jailbreaking isn't about picking the average iPhone user off the street and trying to convince them to do it to their phone. For a somewhat savvy 'nerd' that wants a little more from their iPhone that can understand what is being done, it works.
Apple's security blanket is nice for the average user. Nothing mysterious can be installed, and no problems really can arise. If you put a Jailbroken phone into the hands of a complete noob and idiot, sure, there can be a potential for a (quite) corrupted iPhone. Although, a simple software restore in iTunes will fix all of the problems.
