Apple has released iOS 4.0.2 for iPhone/iPod touch and iOS 3.2.2 for iPad to patch the PDF exploit that JailbreakMe.com has been using in their current browser based jailbreak. Meaning that if you’re planning on jailbreaking anytime soon, don’t upgrade your iDevice past 4.0.1 or else you’ll be sunk.
Never fear though, if you failed at using commonsense and have already upgraded, the jailbreaking community is hard at work on a new jailbreak for 4.0.2, although it’s likely this one will end up being a tethered jailbreak (in other words it will require a computer in order to jailbreak your iDevice).