>>17
Well, if it makes you feel any better, I can't think of a time I'd ever use が instead of は in that sentence. I'm quite sure there are good times to use it, but while I learned the difference between は and が, I couldn't tell you why to use が there.
I recall は is used when contrasting things and when introducing a new subject/topic thing. E.g.,トムはアメリカ人だ。博仁は日本人だ。 is the "contrasting" example.
I suppose if you're already talking about yourself, you could say 私が日本人だ, but in that case wouldn't you just say 日本人だ? I'm a non-native speaker, but I like to think my は vs が usage is instinctual at this point.
Perhaps not...