The original iPhone users have, for the past half a year or so, been privileged to know the benefits of jailbreaking. These benefits, or namely the benefit of being able to install third party applications that aren't available anywhere else are really the only reason to own an iPhone these days. With the unlock, several options become available - you'll be able to switch providers assuming you yank out the AT&T-provided smartcard and replace it with a different providers'.
Now, let's quickly go over the install process, which is incredibly simple:
1. The yellowsn0w app will look and run exactly like the iPhone 2G Unlock
2. Just follow through the first couple of prompts until it asks you what to install or unlock
3. Make sure to install the cydia repository rather than the installer repository, since it houses most of the best apps
4. Now, follow the button pressing prompts TO THE LETTER. The program will ask you to click a series of buttons in a very specific order - go ahead and do that exactly. Don't worry, if you fail, you can always go back and try again.
5. Assuming Step 4 went smoothly, yellowsn0w should now be installing the Cydia app onto your iPhone and potentially unlocking it
6. Once unlocked, disconnect your phone, go into the Cydia app that should be installed now, and let it update
7. As soon as the Cydia app updates and reinitializes the iPhone, you're ready to start browsing for cool new third-party apps