For what it's worth...
I ran into my OB (disclaimer: is very crunchy but also loves her tea, lol) at the coffe shop this morning, getting her tea. I asked her specifically about green tea. She said that she strived to keep up on the research here. To her knowledge, there has been no definative studies done that prove adverse effects. She said the main concern is that green tea contains EGCG which has a recation with the enzyme DHFR. Something about this reaction is speculated to inhibit folate absorbtion. She noted that in the research she has done, the amount of green tea that needs to be consumed is much higher than most anyone would every drink, the even start to cause a problem.
She said her general advice would be to keep in to a cup a day or less (if paranoid) early in pregnancy until the neural tube closes. Then keep it to a couple cups a day if the caffiene doesn't bother you.
Her two cents (as an M.D., but also a super lover of green tea).