iOS update 4.0.2 is available on the iTunes Store. This firmware 4.0.2 fix the weaknesses of the PDF Viewer on the Safari browser. This weakness was exploited by hackers to Jailbreak iPhone 4.0 and 4.0.1 IOS via Safari.
With this 4.0.2 hackers have to think again how to Jailbreak iPhone. My advice to iPhone users, do not update OS 4.0 and 4.0.1 to 4.0.2.