I almost feel obligated to put in my two cents here, as both someone living poly and as someone with bipolar disorder.
Now, among our friends I am both openly poly (I don't really advertise, but I don't hide anything either)' and openly bipolar. That said, of all the people I know, I have a handful of both diagnosed bipolar and poly people. As far as I know I'm the only one that's both poly and bipolar. I will make a note and say That when I refer to someone as bipolar, I'm talking people with a diagnosis, not people who assume or heard that they might be and call themselves it (something I take offense to, for my own reasons).
I'd also like to note that bipolar is very different then depression. Depression can often be treated by some medication and a change in situation, bipolar never, ever goes away. It can be treated, handled, and and dealt with, but it never, ever is cured. It doesn't just come on because you ate depressed, there is more to it. I think part of why it's everywhere is that it's often used as an easy diagnosis, and far too many people are over medicated, kind of like depression. I'm betting that there are a lot of depressed people diagnosed as bipolar and vise versa.