This is actually a very fascinating topic. Fascinating not so much because we can't figure out the financial side of things. The problem here is how does one go about defining and perhaps even quantifying happiness? Can there even be a quantitative threshold upon where you reach "peak happiness"?
And to stir things even more, I also think our standards and definition of happiness changes as time goes by as well. For example, when I was younger, I was completely ignorant about money. And yet, for the most part, I was fairly happy. Now that I know a lot more about money, I admit I'm not too happy about having been so broke and ignorant.
I suppose that brings up another issue, eh? Happiness through ignorance. One can be happy because they don't know any better. The downside, of course, is that you are still in peril. You just don't know it... but because you don't know, you can be happy for a while (until the unforeseen danger materializes).
So... in the end, it is what some of you have pointed out: That happiness is about perception somehow. My teenage self would have been ecstatic if I had just $100 in the pocket. Now? I wouldn't even blink an eye at $10k.
However, I don't know what the exact answer is. I too have read several studies that tried to quantify a certain income threshold as where happiness peaks. But truth to tell, I think that's barking up the wrong tree. Wherever the answer lies, I think we need to keep focusing on the concept of happiness itself....