The BlackRa1n jail break has been released for the iPhone 3G/3GS and is available for download here. Currently only Windows is supported. LoL.
Please note that this jail break only works on phones upgraded from iPhone OS 3.0 or 3.0.1 or iPhones which are SIM unlocked. This release will not resolve baseband upgrade issues or hacktivate+jailbreak new iPhone 3GS units with 3.1 firmware installed as the factory default.
Once jail broken, install Cydia, Rock or Icy then add this repo to your source list: http://iphoneinthailand.com/cydia
As always, be advised that jail breaking will void your Apple warranty.
Hurray for Geohot.