Jailbreak a iPhone 3gs running 4.2.1 preserving BaseBand?
It looks like you are lucky. First of all you should get a utility called TinyUmbrella (http://thefirmwareumbrella.blogspot.com) and save your SHSHs which will allow you to install older firmware on the device, in case you manage to save them before Apple stops signing the SHSHs. Next, see http://jailbrea.kr to tell wether you can jailbreak or not. Lastly, find an apropriate guide to jailbreaking your device via http://iclarified.com