the iPhone DEV team has released redsn0w software which is a simple to use MAC and PC software, similar to QuickPwn, which will jailbreak and unlock iPhone 3.0 software (limitations apply, read notes below).
As of right now here is what you can and cannot do with redsn0w. Read carefully before using redsn0w!
iPhone 1st Generation
Everything! redsn0w will both unlock and jailbreak your 1st generation iPhone. It will install Cydia and will be able to change SIM Cards to a different carrier. This is only for 1st gen iPhone users.
iPhone 3G
redsn0w can only jailbreak your iPhone 3G, but not unlock it. This means you can get Cydia but you won’t be able to use another carrier with a different SIM Card. If you to unlock your iPhone 3G with the new 3.0 firmware, you should wait for ultrasn0w which is coming soon. As a side note, yellowsn0w is able to unlock older 2.x firmwares of the iPhone 3G.
iPhone 3G S
Nothing! Unfortunately, redsn0w can neither jailbreak nor unlock the iPhone 3G S. Wait for ultrasn0w.
iPod touch
redsn0w will jailbreak both 1st generation iPod Touch and 2nd generation iPod Touch.
To use redsn0w simply upgrade iTunes to 8.2, the device in iTunes to firmware version 3.0 and run redsn0w to activate and jailbreak the device (and if you are using an original iPhone 2G, it will unlock it too!)
Official Torrents
redsn0w for MAC
redsn0w for PC
Other files you will need:
Bootloader files V3.9 (BL-39.bin) and V4.6 (BL-46.bin)
Firmware 3.0 for your Device
iPhone 3G S Firmware 3.0
iPhone 3G Firmware 3.0
iPhone 2G Firmware 3.0
iPod Touch 2G Firmware 3.0
iPod Touch 1G Firmware 3.0
Here is a video demo of using redsn0w on the iPhone with Windows
Use iPhone mods and hacks at your own risk! Installing 3rd Party iPhone apps requires you to jailbreak the iPhone before installing the native software and apps. Browse the full list of all iPhone and iPod touch native 3rd party apps filed under the software category.
More info on redsn0w here