In order to jailbreak and unlock an iPhone 3G, 3GS or iPhone 4 with 4.2.1 firmware, you need to be aware of the following information.
If you are using the newer version of iPhone 3GS, then you need to use untethered jailbreak, as this is the only available solution.
This new unlock process updates the existing baseband to the newer version, i.e. 06.15 version. Once you have updated your iPhone to this newer version, you can’t downgrade to an older version. Also, as soon as you update to the newer baseband, you will void the warranty on your iPhone as Apple will come to know about it.
In future, if any critical updates arrive, then you also need to update to the newer baseband version i.e. 06.15. So, there is no harm in updating it now, unless and until, you are not looking to update your iPhone in future.
With newer firmware versions starting from 4.2.1, you will not be able to restore your iPhone to stock firmware, and you will have to use a custom ipsw file for the same.
To unlock your Jailbroken iPhone 3G, 3GS or iPhone 4, read this tutorial How to Unlock iPhone 3G, 3GS or iPhone 4 on 4.2.1 firmware.
Before proceeding further, I assume that you agree to the above mentioned information. To jailbreak and unlock a 4.2.1 firmware based iPhone 3G, 3GS and iPhone 4.
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