I wrote a hub about the health benefits of Green Tea (link at the bottom of my answer), but what you've read is true- kind of. I'll explain further:
The reason for this is no different than the reasons that ANY green leafy vegetable will do this to you: Vitamin K (Called K for "Koagulations" which is German for Coagulation- The thickening of blood) (http://en.wikipedia.org/wiki/Vitamin_k)
Green Tea comes from the same plant that all other teas come from Camellia Sinensis, which is a green leafy plant.
This is why people who take Coumadin (a blood thinner) cannot eat green leafy vegetables. (http://www.drugs.com/coumadin.html)
So, drinking large quantities of Green Tea will be just as dangerous to you as eating large quantities of green leafy vegetables like salad or spinach. Unless you have some type of condition that warns against eating large amount of green, leafy veggies, then you should be OK.
And, as I'm sure you know, too much of anything can have negative consequences, but I imagine most people don't consume so much green tea that this will be an issue of note.
My Green Tea Nutrition Hub: http://hubpages.com/food/green-tea-nutrition