The speculation that arose among all the blogs about the AppStore app KakaoTalk Messenger having a vulnerability that would allow jailbreakers to exploit iOS 5.1 has been deemed as a hoax by the one who initially tweeted about it (@i0n1c).
Here’s what he tells us:
iOS 5.1 is out.There was never an exploitable hole in KakaoTalk, as I said from the 1st day.It was an experiment to see if Apple would kill an iOS app just for the rumour of a vulnerability.We needed to see if using apps for JB is feasible or if Apple will act if we just tell you to download a specific app a few weeks before releasing an exploit for it. The result of the KakaoTalk experiment was that KakaoTalk developers were trying to get into contact to fix the non existant vulnerability. Of course it is unknown to me,if they got some pressure from Apple. We will never know.
After this he also tells us why out of all the AppStore apps did he pick KakaoTalk to become the guinea pig:

Well reading that was a bit disappointing because I was hoping to update to iOS 5.1 expecting a jailbreak anytime soon, but after that I think I’d probably hold my thoughts of upgrading to iOS 5.1.
What are you going to do, wait for the jailbreak exploit or not?
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Posted by iosjedi on March 8, 2012
http://iosjediblog.com/2012/03/08/kakaotalk-messenger-exploit-a-hoax/