Originally Posted by yiming
is it possible for a firmware/OS to be so "well" written that it can't be jailbroken?
In principle, yes - even if that is *very* difficult. Essentially, it would mean that there is absolutely no "exploitable" bug anywhere, in any of the code.
I'm actually convinced that such a bug exists, somewhere, also in 5.3.0. The difficulty lies in actually finding it. Mind you, I did find multiple different bugs of varying severity* in 5.3.0, but none of them is severe enough to be used as a jailbreak vector.
(*) No, I'm not going to disclose them publicly. But don't worry: these things are being discussed internally, in a small circle of expert developers.