Great thread, I think people can get a bit excited about the idea that bitcoin is some new currency that nobody knows about, and that simply getting in early will gurantee a profit of some sort. Investing/trading has all the pitfalls of any other type of currency.
Personally my interest is mining, but it's hard to say what kind of investment i have made. I recently brought a 5850 for AU$169 so I guess you could say that is the amount invested
However it's a bit convoluted because I don't have a dedicated mining rig and I also use the card for gaming. I suppose the extra wattage draw while mining is also part of the "investment".