These are instructions on how to jailbreak your iPhone 3GS and iPhone 4 using Greenpois0n for Mac. Greenpois0n has been updated to support the iOS 4.1 and iOS 4.2.1 firmware. Use Limera1n if you want to jailbreak iOS 4.0, iOS 4.0.1, iOS 4.0.2.
Remember: Greenpois0n recommends a clean restore before continuing. Do not update to iOS 4.1 or iOS 4.2.1 if you need the unlock. Ultrasn0w does not support that baseband yet. The only exception is the iPhone 4. You can update to iOS 4.2.1 without upgrading your baseband by using TinyUmbrella. Start TSS Server->Start iTunes->Plug in your iPhone->Restore 4.2.1->Error 1013->Put phone into DFU Mode->Launch Greenpois0n. We will post more detailed instructions shortly.
Open your web browser of choice and navigate to http://www.greenpois0n.com. Click the Mac link at the bottom of the screen.