Just to add - not sure if this is true or not.
I read somewhere that the dominant white gene that makes white cats deaf by affecting the ear has a special way of displaying itself. As I understand, it's the same gene that creates the piebald pattern. I read that the effect of the while begins in the lower body parts and then moves upwards. So we are more likely to see cats with a white tummy and some color on their back, tail and top of the head. The reverse is usually not seen (a cat with a colored tummy and white back). If the gene displays itself all over the cats's body, including the very top of the head and tip of the tail, then it may or may not move on to affect the eyes. If it does, then we get a white blue-eyed cat. If it doesn't, then we may have a white cat with greeen or yellow eyes. Only then does it move on to effect the ears. If it hits the ears, then the cat is also deaf (as Liz explained, it effects a pigmented part of the hearing mechanism in the ear). So, you could have a blue eyed white that isn't deaf, but if the gene has gone as far as the eyes, you also have a chance of its having affected the ears.
Sometimes you get odd-eyes cats, where one eye is blue and the other is yellow. This means the gene had affected the side of the head with the blue eye, and possibly the ear on the same side as well. However, as it didn't effect the side with the yellow eye, the ear on that side should be ok as well.
This is what I've read a long time ago - not 100% sure it's true - but it did make sense at the time
They also said there are 3 types of genes that can create a totally white cat - so when you see a white cat you can't be sure it's one of the genes that may also cause deafness.