Because he has discovered that Apple has placed fixes for any vulnerability exploit holes or any attempts to go there in iOS 5.1 Beta 3 which just recently released and keep Siri for iPad 3 clue deep within.
This is an Apple's blatant effort to counter Corona who managed to exploit vulnerabilities in iOS 5.0.1 and bring untethered jailbreak to A4 last year.
Nadeem is a iPhone developer and also a hacker who used to create Pwnage Bundle for jailbreak community, and you find its twitter account here and here is its tweet regarding this matter.
There is no more delay on this, be sure to immediately upgrade to iOS 5.0.1 before Apple stop signing it. If that happens you will be eating your finger, when A5 untethered is finally released to bring jailbreak to A5 device untetherely.
And then install Corona via Cydia or untethered jailbreak iOS 5.0.1 with Redsn0w if you have not done so. Don't forget to save your SHSH blobs using iFaith V14 for Windows or for both using TinyUmbrella 5.10.06.