I see that many people still do not understand what this new decoding solution means, so through this article I intend to answer your questions so as not to end up in the comments with the same answer written 10 times. Before reading this article, please read the article first this then this!
iPhone 3G
If you have baseband 05.14.02/05.15.04 you will not be able to decode with the new version of ultrasn0w from Cydia. If you want decoding, you must follow this tutorial. You can jailbreak with redsn0w 0.9.6 beta 5 over another jailbreak made with redsn0w, but not over the one made with sn0wbreeze or other jailbreak programs. Using that tutorial you will not restore but only jailbreak with update to the 06.15.00 iPad baseband which can be decoded by ultrasn0w 1.2. It doesn't matter if you are on iOS 4.1/4.2.1 because the jailbreak will be untethered.
Is it worth updating to iOS 4.2.1?
Yes, iOS 4.2.1 brings some improvements, so it's worth it.
Do I lose applications if I jailbreak?
No, you don't lose anything if you just jailbreak and update the baseband.
Lost activation?
Don't get lost!
If I install the new ultrasn0w from Cydia what happens?
If you have baseband 05.14.02 or 05.15.04, absolutely nothing will happen. If you have a smaller one or 06.15.00 then your phone will be decoded.
iPhone 3GS
Absolutely everything I wrote above applies with the mention that: if you update to 4.2.1 (and you have a new bootrom) you will have a tethered jailbreak, information about this you can find here.
iPhone 4
Unfortunately, there is no decoding solution for this terminal. The jailbreak for iOS 4.2.1 is tethered and can be done with this tutorial.
If you have other suggestions, I can complete the list of questions and answers.
Please pass it on.
