Why you may ask? At Pwn2Own contest, iPhone 4 was hacked via vulnerability which was found in Mobile Safari running iOS 4.2.1. This vulnerability also exists in iOS 4.3 but all devices running the latest version of iOS are safe until hackers find a way to bypass the ASLR (Address Space Layout Randomization) which Apple has implemented for all supported devices starting from iOS 4.3.
taken from here.
Jailbreak iOS 4.3.1 Untethered To Be Release Once Apple Releases iOS 4.3.1 To Public | Redmond Pie
