spenceR san,
Sorry, my japanese PC is down... (I'm so pissed!!!)
If your baby is born in the U.S, he/she will be American by law of course, but if born in Japan I beleive that all you have to do is report it to the US embassy since you (the parent) is American.
I have US and Japanese citezenship, my husband is Australian and got US residency through me, we are thinking of having our baby (in the near future) in Japan and of course because she is born from me as a Japanese, he/ she will be Japanese (watashi no koseki ga aru kara) and when I report it to the US he/she should be able to gain US citizenship since I am American also. I am thinking because my child will have an Australian father, he/she will be able to obtain Aus citizenship as well!!!(how cool is that!?)
Hope this helps.