Apple released their latest firmware update for both the iPhone and iPad today, versions 4.0.2 and 3.2.2 respectively. Unfortunately for those who haven't jailbroken their device yet, this will prevent you from using the PDF based JailbreakMe tool in the future. In a similiar fashion, it will remove the jailbreak status of your iPhone 4 and iPad if you update the firmware, a standard procedure for past iOS updates.
Apple should receive a little credit for playing the responsible parent, because the same method used to jailbreak an iPhone can also be used by a malicious hacker to compromise the security of your device. Savvy users should already be aware of the app, PDF Warner,from the Cydia store, which does the same thing, but allows you to keep your device jailbroken, and jailbreak it down the road. Novice jailbreaker's would be advised to ignore this update.
PDF Warner is the advisable solution. Apple hasn't fixed anything other than the PDF issue, and until they do improve something useful, like the proximity sensor, it seems unwise to update at this time if you value your and your device's freedom.
Apple did a great job in security of their products. Protectionism is a core element of the iPhone's success, in Apple's view -- but ultimately, this might come out as a decision that's difficult to defend, as it’s legal to jailbreak like you have done for you iphone 4, tutorial like this “Jailbreak/unlock iPhone 3GS for iOS 4 on Mac”, posted in ifunia iphone column.