Just a few other thoughts, from my experience of running numerous voice workshops and from performing ...
1) Nervousness and stage fright produce a degree of deafness to pitch. I can't tune my guitar accurately if I'm nervous, nor can many other performers I know, and for some people that results in singing out of key as well without realising it. I've even heard albums where the singers are plainly out of tune but this hasn't been picked up by the engineer, the producer or the singers.
2) It's actually quite hard for quite a lot of people to hear the pitch from certain instruments, such as the guitar - maybe it's the frequency, maybe it's familiarity, but either way that's what happens.
3) A huge amount of music written for children is written on the (false) assumption that children sing soprano. Not all children do, and those that don't have enormous trouble finding the right note in a register they can't easily cope with. Which leads to the "I can't sing" syndrome and, frequently, the identification with tone deafness. And it's a vicious downward spiral! The more you think you can't sing, the more you can't. And if as a child you knew you "couldn't", then it's even tougher as an adult.
And 4) on Pop Idol and other similar shows the auditions are for singers to perform a capella, which is difficult even without audition nerves.

Not a lot to do with tone deafness there, then, in my experience!

Anne (Lister)