Is there any written software by any industry that is not hackable?
It largely comes down to minimising the attack footprint and validating inputs correctly.
It also depends on your definition of "hackable", e.g. whether you include Denial of Service attacks, or hard-core chip-level firmware attacks in your definition.
But I would say that if you look at software that was formally designed and tested, written in something like Ada or Spark, and where a minimal interface is provided to local and and remote users ... that sort of thing is probably nearing "not hackable".
In the end, the best IT security is layered security ... and anything critical should be protected from the outside world by an air-gap or data-diode.
If these hackers (where ever they come from) can get into Sony surely that will leave tablets phones DVD players TVs etc vulnerable to attack when customers upgrade software?
I've heard rumors that the hackers stole Sony's code signing keys, which would mean they would be able to sign software and make it look like it was kosher and came from Sony !