Last month, I jailbroke and carrier-unlocked my first-generation iPhone. I followed this iClarified tutorial. Near the end I got an "error 1600," which I was able to rectify by following the instructions found near the end of this MacRumors post.
No biggy. I can't say I wasn't a little nervous, but each time something went wrong, I just turned my phone off and on and it was back to where I'd started. After a few restarts, I became reassured that I wouldn't end up with a brick on my hands, so long as I was careful following the instructions.
After my eventual success, I became a staunch advocate of jailbreaking and unlocking (the latter currently only available on the first-gen iPhone, though the folks over at iPhone-dev appear to have finally broken through the 3G's defenses). For the coders among you, you get a cool terminal:
But, perhaps more useful, with the unlock, any SIM card from anywhere in the world will work. That's how I was able to use my iPhone with my Movistar prepaid SIM when I went to Spain. Of course, their data charges were abusive, and there's no 2.5G (aka EDGE) in Spain as far as I could gather, which substantially diminished the usefulness of the phone, but at least I got to keep my phone and iPod in a single device.
An additional advantage of jailbreaking, by the way, is the awesome PDANet, a tethering application that allows you, with minimal setup, use your AT&T (or whatever) internet connection with your laptop—much nicer to type an email!
So, a couple of weeks ago, I'm telling my friend that he should unlock/jailbreak, and he tells me about his friend's experience with the jailbreak: his iPhone had problems and the nice folks at the Genius Bar told him that they couldn't help him because his phone had been jailbroken.
You can imagine me getting worried today as I notice my phone has a problem: I can't get any sound whatsoever from the built-in speaker—only through the headset. Turning it on and off and re-synching did nothing.
Well, I am happy to report that "undoing" the jailbreak is easy as pie: go to iTunes while your phone is connected to your computer, hit "restore," and whenever it asks you whether it should use a backup, say no. This will restore the iPhone to factory settings, getting rid of all your jailbroken apps and so on. The phone itself will actually remain jailbroken, but not in any way that an Apple Genius can easily determine. If you head to the Apple Store, you'll get the same service as all those non-jailbreakers!
Anyway, you might be hoping to hear how my trip to the Apple Store went, but it's not gonna happen this time. I ended up finding the solution to my problem in this Apple Support message board.