Black vs. white backgrounds (also, chalk)

(No, this post is not about race).
The other night, I and my friend 4pq1injbok were discussing the compelling, overarching, enormously important issue of black text on white ground vs. white text on black ground. (Woo, having spare time!)

Of course, each option is appropriate in different contexts. If you're printing something out, you better have a good reason to do it in white on black, because that wastes so much ink/toner. But for viewing on screens (including projection screens), I vastly prefer black on white because it reduces glare and seems to make my eyes less tired. Mostly the former. Glare is a problem especially on very large projection screens: you can dazzle (in a bad way) an entire lecture class with a white-backgrounded Powerpoint, especially if the projector is having an off day. Not fun early in the morning.
What troubles me, then, is that there seems to be some kind of unspoken rule in academia that you can't use black backgrounds, and I'm not entirely sure why. I watched a dry run of a job talk a couple summers ago, and in the midst of everybody commenting on every facet of every slide, my mentor explained to me that there's just a certain way scientific presentations are done, which includes black text on white ground. The Google example is telling, in one sense -- Google is renowned for the "clean" look of its pages, which I'm sure has a lot to do with being white-backgrounded. Perhaps there's a lot of pressure from older academics who are more comfortable with print on paper. Perhaps, also, it makes pictures easier to see, but you can fix that with a relatively narrow white border. I think we should take a hint from the movie industry; when was the last time you saw the end credits in black on white?

(Also, using black backgrounds probably saves energy -- I don't know any numbers, but the savings must be considerable for large screens. There's even Blackle, an almost entirely black version of the Google homepage. Supposedly if everyone used it, we'd save ~750 MWh a year (hat tip for the calculation to ecoIron).)

Chalkboards are the only common medium I could think of where the default is light text on dark ground. (Not to say there aren't others; it's just that I couldn't think of them.) Nothing profound about it; chalkboards are this way because CaCO3 is white, and slate is dark gray. But it does raise an interesting question. Suppose you draw two circles on the board, and fill one in with chalk while leaving the other one empty. Which is black, and which is white? Do most people just seem to naturally agree, or is there some kind of consistent convention, or is there no consistency at all?
In my music theory class, and anywhere I've seen musical notation on a chalkboard, the convention is that quarter notes are filled/white while half and whole notes are empty/black. In print, it's the other way around, at least with regard to absolute colors. But in this case, you can define it in terms of filling, not color, so it's a bad example. I'm rather tempted to crash a game theory class on the day they discuss chess/checkers, or some kind of visual arts/design class on the day they discuss positive vs. negative space, or take a poll of professors, or some such.
Related linguistic issue: on the Improbable Research blog, a prof rants about the suckiness of Crayola's new chalk, complaining that "the new pieces are thinner, shorter, and don't write as dark", using "dark" to denote degree of whiteness. But "don't write as well" lacks specificity, and "don't write as lightly" would be interpreted the wrong way around. Sounds like "heavily" (or perhaps "cleanly") might be the way to go.

And finally (and randomly): hooray for profs who know how to use colored chalk well, to highlight information and obscure noise, without overusing it. More on this subject if I ever get around to reading Tufte's books.