UPDATE : Since iPhone 3.0.1 is a minor upgrade, it is still jailbreakable and unlockable. Please stay away from 3.1 firmware when released, it can be a disaster otherwise!
All you have to do is :
1) Update your iPhone to 3.0.1 firmware via iTunes
2) Use redsn0w to jailbreak. While using redsn0w – when prompted to choose the firmware, simply choose 3.0 firmware instead of 3.0.1 firmware and it should work just fine! This works with iPhone 2G, 3G and 3GS.
3) Enjoy your iPhone!
As usual, stay away from the update for a day or two while the firmware is properly patched for jailbreaking and unlocking. Baseband is not updated, meaning unlock still works. Jailbreak supposedly works too but until further confirmation, stay away from the iPhone 3.0.1 update.
This update patches a critical SMS bug which supposedly can break networks, hijack your iPhone and end the world!!