Thanks for all the replies so far. The study mentioned by hueyracer in his post certainly sheds some light on the subject.
I asked the original question as I had never actually heard of the condition before. Whilst I didn't doubt that it existed I was curious as to how widespread it really was.
For me the most interesting aspect is that more experienced and older pilots are apparently more susceptible to the condition. At least that is what I gleaned from the study hueyracer refers to.
Is this an accurate assessment in your personal experiences? That is, has anyone started to suffer from it later in their career, after several years of having no problems?