JailbreakMe 2.0 when run from your iPhone or iPad allows quick and easy jailbreaking. Jailbreakme uses an exploit in Safari to run the jail break code (which means it's unlikely to work forever - at least until the next update). Lately there have been quite a few articles floating around about how easy this specific jailbreak works and some of the downsides like in Cydia there are no applications that take advantage of the Retina display on the iPhone 4.
In my post The Case for an iPad I toyed with the notion of jailbreaking an iPad to release more functionality like installing other browsers (Firefox and Chrome), using flash, etc. and although I don't have one to try it on, I think this is a step in the right direction.