Here is a nice little article on the history of the 440 hz basis of pitch.

Jon, the modern piano is tuned in equal temperament as opposed to well temperament. In equal temperament the ratios of frequencies are the same for the same interval in any given key. In well temperament this equality is compromised in favor of better harmonic purity. Actually, it may be more appropriate to say that equal temperament compromises harmonic purity for equality.

PA