After the successful iOS 4 jailbreaking by @comex through Jailbreakme.com, Apple has released a new update which patches the PDF exploit used by Jailbreakme.com to jailbreak iPhone and iPod Touch devices. The exploit which was used by @comex could also jailbreak the new iPhone 4.
If you rely on the jailbreak, it is highly recommended to skip this update since you will not be able to jailbreak iPhone or jailbreak iPod Touch once the new update is applied. This update will also the unlocked iPhone useless, so stay away from it till an update is available from the dev team.
(Source: Redmond Pie)