I think the soy debate is very interesting, and the bottom line is that NO ONE REALLY KNOWS. They know that there are phytoestrogens in soy, but they're not sure if they inhibit or promote our own natural estrogens. Then there is the issue that there are so many different agents in soy (some seem to be anti-cancer) that it is hard to study the effect of any one specific component of it and moreover the complete effect. For every study that shows that soy inhibits thyroid function, there are three more that shows it reduces the risk of alzhiemers and cancer.
I frequent an environmental endicirnology lab at my university, and, for whatever its worth, the people there, including a man who is basically one of the founding fathers of environmental endicrinology, are big proponents of soy. I personally am a bit skeptical.
My advice would be to consume soy in moderation. Everything in moderation is an important principle which Asians seem to know well. I feel their general philosophy and lifestyle lies at the root of their health more so than the amount of tempeh they consume.