My college housemate is now a professor of music at Mercer College; I wrote him and asked about this question. He wrote back:
Scott,
Your question has me thinking. . .
Probably relates to Guido, the Swiss Monk/choir master at Arezzo Monastery, who taught sight-singing by assigning note names to the joints of his fingers and other parts of his hand: the Guidonian Hand. This utilized all the notes in use at the time, which spanned from low G (Gama) all the up to high C (Ut). Thus, all the notes were known as the span from Gamma to Ut, which was later shortened to Gamut, which is still popular for everything else, though rarely used in music. These of course were all white notes, except for Bb which was added to help avoid the tritone (Diabolis Intervallis) between f and b, which the monks had trouble singing anyways.
So, chromatic pitches were added throughout the Renaissance, but there were not really keys as we think of them, but rather modes, and the theory for composition was more linear and harmony was modal. Modal harmony has chords, but they are more coincidental and result from multiple melodic lines interweaving following rules for counterpoint.
The big change came right around 1600 with a concerted effort by a group of modernists who met at the Count Bardi's palazza in Florence (Florentine Camerata). While their main interest was in a new style of opera, where the text governed the music (or at least wasn't obscured by the contrapuntal lines with multiple simultaneous bits of text competing with each other). The new homophonic style required key signatures. The Ionic mode was contrasted with the Aeolian mode (major and minor). Using the notes from the Gamut, C is the only key with no accidentals. G is probably the most popular key for music.
That's my historical mish mash off the top of my head. Let me know when you learn something else to correct, or add to the story!
With the tip off about Guido, I was able to find several good articles from both music theory- and religious-related sites, including:
http://www.canticanova.com/articles/misc/art7af1.htm
The Wikipedia article on Guido led to this article:
http://en.wikipedia.org/wiki/Guidonian_hand
Here is a sample of the Guidonian Hand and some information on the hexatonic scale (not the pentatonic scale, like I speculated above):
The idea of the Guidonian hand is that each portion of the hand represents a specific note within the hexachord system, which spans nearly three octaves from "Γ ut" (that is, "Gamma ut") (the contraction of which is "Gamut", which can refer to the entire span) to "E la" (in other words, from the G at the bottom of the modern bass clef to the E at the top of the treble clef).
Interesting to see yet another version of TAB, which I am considering switching to from now on when posting (not).
From this reading, it appears to me that the two pitches from which all eventual scales were referenced were C and F. In the Gamut, C was the scale with no sharps or flats (arbitrary but not capricious?). I still haven't found out what the priests and monks used to make those reference pitches in the different places they lived.
So, at this point, it seems like it was likely just happenstance, as speculated. Interesting to see how many of our spekyulashuns actually have some relationship to conventional wisdom.
Here is one cool Youtube video / podcast on Guido. I'd like to visit this Italian Music Museum some day.
http://wn.com/guido_of_arezzo
