You really should update, unless you already secured your iPhone via a jailbreak solution.
Quickly after the hype of the super easy jailbreaking via JailbreakMe security concerns in the mobile Safari browser of the iPhone become center of attention. Hackers in the jailbreak community offered a fix days before Apple released iOS 4.0.2 to make your iPhone secure again.
I just downloaded it today, as I was too busy with our relaunch. If you have not upgraded your iPhone 4, iPhone 3GS or iPhone 3G yet, connect your iPhone to iTunes now.
Of course jailbreakers avoid this update. More details about the iOS 4.0.2 iPhone update can be found on the Apple site.