Both the members of iPhone and Chronic Dev Teams have confirmed that the creator of Spirit and JailbreakMe, Comex, has more userland exploits in possession which can jailbreak iPhone 3GS and iPod touch 3G in future. But dont expect him to release them anytime soon. Since userland exploits can easily be patched by Apple, they are going to wait for Apple to release the final version of iOS 4.2 before releasing the new userland based jailbreak.
Also, the GreenPois0n tool releasing on Sunday is based on SHAtter exploit which was originally a tethered jailbreak. Comex came up with a userland based method to make it untethered in GreenPois0n tool. What this means is that Apple is likely going to patch the bits done by Comex in GreenPois0n which will make GreenPois0n a tethered jailbreak for all the supported devices on iOS 4.2.
SHAtter is yet another breed of exploit. Bootrom-based but requires userland "assistance" for untethered boots (@comex did this for 4.1)
The wait will then be for this new userland based jailbreak by Comex which will not only support iPhone 3GS and iPod touch 3G, but will also make GreenPois0n an untethered jailbreak again for all the fourth-gen devices on iOS 4.2.