The Dev-Team has made a few more updates to their post announcing the redsn0w unlock for iOS 4.2.1. One of the updates includes an unofficial PwnageTool bundle for the iPhone 3GS. This update is specifically for people with the iPhone 3GS who would like to upgrade to iOS 4.2.1 but want to keep their current baseband.
Update #3: 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
See the Dev-Team’s full post HERE.