Jailbreak a iPhone 3gs running 4.2.1 preserving BaseBand? Is there a untethered way to jailbreak an iPhone 3GS running iOS 4.2.1?

The phone has never been jailbroken before.

I need to be able to reverse it, so I am hoping that there is a way to do this without upping the 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