I've read many proponents of a New Age tenent that A440 is a "wrong" or "unnatural" tuning frequency, instead insisting that frequencies such as 432Hz would be better. While this is a cute and well-argued point, it has absolutely no merit in an acoustical and musical sense simply by looking at musical history, basic high school physics, and by doing a few practical experiments. If you find anyone of this disposition, link them here!
Back in the Renaissance, instruments had all sorts of tunings- anywhere from A380 (an english pitchpipe) to A480 (a German organ) could be found in abundant use! Slowly this consolidated down to the main few we see today- 435, 440, 442, and 443. Other cultures use other tuning frequencies sometimes too. 432 was never really a common tuning (a basic google search can show you this) and is actually NOT the same as C256 (it's actually C258). Instead, 415, 435, 438, 440, and 442 are the most common ones in history and to the modern day.
The original decision of 440 was decided in 1834:
: The Stuttgart Conference of 1834 recommended C264 (A440) as the standard pitch based on Scheibler's studies with his Tonometer. For this reason A440 has been referred to as Stuttgart pitch or Scheibler pitch.
This was reinforced twice, by a conference in 1939 and again by the ISO later on in the 20th century. This has absolutely nothing to do with Nazis no matter how much you idiots want to make it sound like it does. The need for a common tuning pitch was important in a fight, but the fight was against "pitch inflation", or the slow raising of the average pitch of music, not any country or other cause. 440 was a great median compromise between the High Pitch average of 445, which was proposed in England, and the French Law 435.
Here's the deal- a note is a note. I can take my trombone and move the tuning slide out so it is at A432, but it will just sound like an extremely out of tune trombone, nothing more, nothing less. I might as well use A415 or A460. The reason this works this way is something called Temperaments, which describe the relation between the notes. You can use this to create any musical scale you desire as long as you pick a tonic, or starting note, for instance, C. In our modern system, we use Equal Temperament, which means you can pick any note in the chromatic scale to start your piece on and it will work as all the half-steps are the exact same size. This supposedly has the downside of removing something called "key color", or textural coloration created by the unique dissonances and consonances of certain intervals found in temperaments in which half-steps are not equal.
300 or even just 200 years ago, ET didn't exist- instead, there were unequal temperaments designed to create contrast and the "key color". Perhaps this is what people complain about music "lacking" these days- equal temperament spoiled the perfection of the triad ever so slightly as a sacrifice in order to make music accessible across any instrument and key- small sacrifices in the name of a huge step forward. Equal Temperament revolutionized music the same way the abacus revolutionized mathematics- it practically multiplied the tools available to composers (key changes, new/larger ensembles with less intonation issues, etc.) and standardized music so any instrument could play anything as long as it was within range (theoretically). It was through Equal Temperament, NOT the change of the note tuned to, that changed harmony to be slightly imperfect.
Finally- the nail in the coffin. Anyone with basic knowledge of how sound and music works will know that the scale is based on ratios, at least, traditionally. You can take ANY pitch and create a scale using the ratios of the notes and it can be heard, played, and resonate properly. Anyone who knows basic acoustics will know that resonance and reverberation are different qualities and are not limited to certain "ratios" or "perfect numbers" for most musical instruments. This can be proven very easily with a guitar or a piano with a string slightly out of tune- if you press that key or pluck that string, it still resonates inside the instrument just as much as if it were in tune! This is why guitarists don't check to see if their instrument is in tune by seeing the notes resonate or not, duhhh. The only way a note will resonate in an situation where other notes would not would be in a place such as the resonators under the keys of a Xylophone- big metal tubes cut and designed perfectly to resonate with the pitch the key above it is. This gives the xylophone and marimba their "ring"- otherwise it'd just sound like you were hitting a chunk of rosewood (or synthetic material). You can make a resonator to any length to match any frequency your little heart desires.
And one last thing- any physics student will know this one! The note you tune to is actually completely irrelevant if the temperature changes. As sound moves differently depending on the temperature of the air, your instrument will sound lower or higher if it gets hotter or colder! Also, instruments made of metal or wood can change as they heat/cool, causing them to change pitch as well. That's why you can "get" in tune while you play if you play a brass or woodwind instrument as it heats up if you were previously out of tune.
Ah, but that's enough ranting for now! I hope at least someone got enlightened by this. Feel free to learn more about temperaments (<3!) here, and more about concert pitch and its long history here. I drew most of my information from those pages and my other studies, practical life experience as a musician and a composer, and someone fascinated in early instruments, temperaments, and tuning practices.