For those Mac users with an old-bootrom 3GS who really know what theyre doing, heres 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 youve restored with the help of that bundle, please use redsn0w. If you dont know how to drop a bundle into PwnageTool.app, please hold off on 4.2.1 until its untethered for everyone
You can find a tutorial on how to install this firmware bundle here.