Is there a jailbreak for iOS 4.2
There is a howto in redmondpie.com for jailbreaking the iPhone 3GS with iOS 4.2.1, but it appears there is no unlock yet.

http://www.redmondpie.com/jailbreak-ios-4.2.1-iphone-4-3gs-3g-ipad-ipod-touch-with-redsn0w-0.9.6b4-guide/