Tutorial: How To Jailbreak Your iDevice on 3.0 Firmware
Link to the dev team blog: http://blog.iphone-dev.org/
IMPORTANT NOTE: If you already have the 3.0 firmware for your device, there is no need to redownload it. Just look around on your computer and find it, and if you have it then you will be good. Otherwise you will need to download the firmware from one of these links, depending on your device. The iPhone 3G S jailbreak is not done yet, so don't even try to use redsn0w to jailbreak it. The dev team are working on this as we speak, but it isn't done yet.
You also will need iTunes 8.2 installed if you plan on using 3.0, regardless of whether it is jailbroken or not.
I recommend that you restart your computer before opening redsn0w, although I've not heard of anyone else with the problem I had it's best to be safe than sorry. If you have any problems with the jailbreak (such as redsn0w hanging) and it looks like your device is bricked, don't worry. If you are sure that nothing is happening, hold the home and power button for roughly 30-45 seconds until your device reboots. It won't be jailbroken, but it should at least be the way it was before.