It is now possible to Jailbreak your iPhone and/or iPod Touch over the web simply by visiting http://www.jailbreakme.com/ in your Safari browser. Once you do so Safari will disappear and return to the Home screen, don't touch anything for a minute or two until the Slide To Unlock screen appears. Once it does, you can slide to unlock and the Installer.app will now be installed on your iPhone or iPod Touch!
Please note the website may be down as I guess many people are trying this, so be patient and it should be back online soon.