Originally Posted by yiming
is it possible for a firmware/OS to be so "well" written that it can't be jailbroken?
In the case of Unix and its derivative and "work a likes" (such as Linux), a qualified: YES.
No system can be secure if the system's operator console is accessible.
In this case, see the indexed information on accessing the serial port (which becomes the system's operator console once Linux boots).
- - - -
The above is the general answer, a more specific answer - - -
Here (at Mobileread) the "jailbreak" method(s) are expected to be run by any end-user, regardless of knowledge or skill levels.
Such a qualification eliminates the use of the serial console.
With this additional qualification, the answer is: "Yes, It can be locked from end-user alteration".
In fact, without even trying very hard.
As a contrast, see how many success stories you can find of anyone "jailbreaking" the OS on the Qualcom SoC that runs your 3G card.
It is also a signed binary, only in that case the signature verification is done in hardware buried inside the chip.
Something the Kindle SoC could also do, but Amazon does not use that feature. Which is what I meant above by "without even trying very hard".