Actually, it's not an American bastardization at all, but the pronunciation is correct as DoxManDude stated earlier.
My source, the unabridged version of the Random House Dictionary of the English Language, gives the language derivation of "forte" as originating from the Middle French. Now, we all know that those Froggs don't pronounce their trailing "e"s (but we also know that the Italians do).
So, to be correct, we should not be pronouncing "forte" as two syllables in the context of this post, but if we refer to the word in a musical sense, then we would say the "ay" syllable, because in music, "forte" derives from the Italian "forte" or strong, from the Latin "fortis".
Class dismissed.
Oh, and in regards to my armchair climbing, I'm at my best in my StratoLounger...