Use TinyUmbrella To Preserve Your iPhone 4 Baseband And Jailbreak iOS 4.2.1 Using Greenpois0n - Unlock with Ultrasn0w 1.2 - MAC

A couple of hours ago, Chronic DevTeam finally released a new update for greenpois0n which allow us to apply an untethered jailbreak to our iOS devices running iOS 4.2.1 ( and iOS 4.1.1 ). Normally, unlockers should avoid this tool, because in order to use it you need to restore to a stock 4.2.1 FW which will update your baseband. But, thanks to TinyUmbrella, iPhone 4 owners who need to unlock, can update to iOS 4.2.1 and use greenpois0n without loosing the ability to unlock.

NOTE: iPhone 4 ONLY

1. Download and install TinyUmbrella ( if you didn’t already )
Prevent iPhone 4 Baseband Update - iOS 4.2.1 iPhone 4 *ONLY*

I've added a page to the blog specifically for helping folks update to iOS 4.2.1 on their iPhone 4 while protecting their baseband. Note, this will not downgrade your baseband. All it will do is PREVENT your baseband from being updated. Enjoy!
links
HELP!
TinyUmbrella [OSX]
TinyUmbrella [WIN]
TinyUmbrella [LINUX]
TinyUmbrella [Core Source]

2. Download iOS 4.2.1
3. Load TinyUmbrella and Start TSS Server. To make sure it’s running, check out the logs…

4. Start iTunes
5. Plug-in your iPhone
6. Restore to iOS 4.2.1
7. You will get an Error 1013. That’s totally normal.
8. Put your iPhone in DFU
9. Use greenpois0n to Down load jailbreak. here... greenpois0n tutorial
10. Finally you can unlock your iPhone 4 running baseband 1.59.00 using Ultrasn0w 1.2, follow our guide