Before I go on with my view I want to be clear, I've never been diagnosed with depression (although I suspect I've been close once) so I'm talking here purely from my personal thoughts without experience. I also think that everyone who is happy to admit that they have had or currently have a form of depressive mental illness needs to be applauded

as we can see there is lack of understanding out there about the situation and the more that are happy to share their experiences the better peoples understanding will be (so

again). Finally before I put my thoughts down I want to say I'll be using the terms depressives and non-depressives purely to describe those that appear to be prone to depressive illness vs those that don't appear to be prone to depressive illness, I hope these terms are OK with everyone.
My Take:
I believe the brain is a fantastic device especially when you consider the millions of things it does on its own to keep us alive, all those things that it does without you needing to be consciously being aware of them; controlling the heart, individual muscles, levels of hormones / chemicals etc, digestion, breathing, repairing minor injuries, the list could go on forever. Even something as simple as lifting your arm and picking an item up really brings home the power of the unconscious mind (think about all the muscles that are moving in that precisely controlled manner by the unconscious part of the brain).
For me there is something that is clearly different between depressives and non-depressives and there is an argument that this is down to chemical balance in the brain, others argue that this is psychological. I believe that it is both, after all it is the brain that controls the levels of the chemicals. I fully accept there is plenty of evidence that the pills work, hence their usage by the NHS. However, I can't help but wonder if this is may just be a temporary solution focused on the symptom and not the cause. There is evidence to support me here because as soon as the depressive is better the pills are stopped and for a while they feel ok but sometimes they slowly slip back down the slop as the chemical imbalance builds. There are plenty of examples of depressives who have numerous bouts of illness. With my belief about the power of the brain I can't help but wonder that with the right treatment why can't the the brain sort itself out, we already know how powerful it is?
To some extent I see this argument behind the comments that a1ex2001 has put forward. In a way he managed to successfully get his own brain to correct the chemical levels, although I recognise that not everyone can do this on their own like he did.
As I said this is my thoughts on it, I am not an expert and I have tremendous sympathy for those that are in that dark place at the moment you will find your path out soon
Anyway back to Petemc's OP, I often find that my best creative photography comes from when I'm looking inward and from what I've read about depression there is a lot of inward looking going on. If what I've read is true then perhaps there is a case that depression is linked to creativity. It clearly isn't a direct relationship as there are lots of creatives who are non-depressives.