Geohot released a beta software for iPhone 3GS jailbreak called PurpleRa1n. It is beta and has some issues, but it gets the job done if you’re patient.
Requirements: You must have
1) Windows xp sp2, Vista, Windows 7, MAC OS
2) iTunes 184.108.40.206
3) Purplera1n software
The jailbreak is a bit incomplete. The kernel vm_mem patch is not present so Winterboard will not work as some other critical tweaks. (SBSettings does work!).
Use with caution and be patient. If you’re not in a hurry, wait for the release from DevTeam.