If you’re a regular jailbreaker, you’ll be happy to know that iOS 4.2.1 is jailbroken already and that you can download the IPSW and roll your own install immediately. RedmondPie has a great how-to but it’s pretty simple.
1. Grab the proper IPSW from Apple.
2. Grab RedSn0w for Windows or Mac.
3. Upgrade to iTunes 10.1
4. Run RedSn0w, select the proper IPSW, and crack your iOS device wide open.
What does jailbreaking get you? Cydia and homebrew software, for one. It also allows a bit more control over the look and feel of the iPhone. If you’re running a carrier unlock, however, don’t try this trick. You could also feasibly brick your iPhone, so cave jailbreaker.
via techcrunch