This just in...
You can now jailbreak almost any iOS device (i.e. iPod Touch, iPhone 3GS, iPhone 4, iPad) by visiting http://jailbreakme.com and swiping your finger across the screen. Dead simple. It installs Cydia in seconds.

Please note: I have seen discussions that this jailbreak may cause issues with MMS and Facetime on iPhone 4. If you experience these problems, just use iTunes to restore your device.