This is cool

Point your iPhone/iPad/iPod touch here:

http://www.jailbreakme.com/

And it will jailbreak any iDevice. I am sure that Apple will fix this particular exploit very quickly so do it before 4.0.2 comes out...

PC Mag article

However, a heads up:

The Really Bad News Behind the iPhone Jailbreak Story

While they haven't issued an advisory, Apple has acknowledged the vulnerabilities being used in the jailbreakme.com site. They say they have already developed a fix and that it will be available in an upcoming update. As reported in the New York Times, the German Federal Office for Information Security, said it had identified two vulnerabilities in iOS. Apple was responding to their reports.

But the big story isn't a vulnerability in iOS, hardly the first or last. This is the first vulnerability on the iPhone to be exploited, although not maliciously, in the wild. [Correction: I have been reminded that the very first iPhone jailbreak was based on an unpatched vulnerability in the libpng [Correction to correction: cmw, who wrote the exploit, explains in a comment below that the vulnerability was in libtiff, not libpng.] library.] The way it works proves that drive-by malware can be done on the iPhone and there's almost nothing to stop it.

Source: PCmag Blog

All jailbreaks are based on zero day exploits which could just as easily be used to install malware. The difference with this one is that it's web-based, so it opens the doors for web based viruses. Which is why I am expecting a patch very soon.

The way they did jailbreakme.com is really nice though - it's just a perfect, one click solution that doesn't even require a restart of the phone. It just works.

Now there's also an unlock for the iPhone 4 so for the first time, it's possible to jailbreak/unlock factory locked iPhone 4s.

So be very careful when buying from grey dealers, and check for Cydia install before buying. If Cydia is on there, I'd not buy it, but if you do only do so at a very substantial discount. The hassle of keeping a jailbroken iPhone running makes it not really worth buying them IMO.