The iPhone Dev-Team has released an unofficial PwnageTool bundle for iPhone 3GS owners with the old bootrom that want to update to iOS 4.2.1 without upgrading their baseband.
For those Mac users with an old-bootrom 3GS who really know what they’re doing, here’s a minimal 3GS 4.2.1 bundle that will get you to 4.2.1 without updating your baseband. Be sure to uncheck "Activate the iPhone" using Expert mode. To actually jailbreak after you’ve restored with the help of that bundle, please use redsn0w. If you don’t know how to drop a bundle into PwnageTool.app, please hold off on 4.2.1 until it’s untethered for everyone
You can find a tutorial on how to install this firmware bundle here.
For those Mac users with an old-bootrom 3GS who really know what they’re doing, here’s a minimal 3GS 4.2.1 bundle that will get you to 4.2.1 without updating your baseband. Be sure to uncheck "Activate the iPhone" using Expert mode. To actually jailbreak after you’ve restored with the help of that bundle, please use redsn0w. If you don’t know how to drop a bundle into PwnageTool.app, please hold off on 4.2.1 until it’s untethered for everyone
You can find a tutorial on how to install this firmware bundle here.