Good topic Bones
I am not sure it is a major issue in aviation, but it certainly is an issue.
Depression, anxiety as well as bipolar are often not understood or diagnosed. Having been married to a person who has suffered severe episodes of depression it is something not well understood.
People with depression find it hard to function every day and may be reluctant to participate in activities they once enjoyed
I don't know the answer but getting professional help and removing the stigma are critical. My wife is now mostly well, but it was a long road, involving professional help (believe me there is a lot of shame/stigma seeing a psychiatrist), medication (for a while) and a lot of family support.
I also know a former airline pilot who lost his licence early in his career... it was a long road back, but he got there.
All I can say is if you think it is affecting you or someone you know see your GP, or followup through one of the excellent support networks like Beyondblue
beyondblue: the national depression initiative and anxiety - What is Depression?
A look over Beyondblue's website is a must!