@zackw – If you roll with the implementation that @chriscoyier suggested, you may also want to consider addressing other visual impairments, like color blindness, which can effect the perception of contrast.
Veerle Pieters has implemented a ‘high contrast’ mode button on her site’s articles which switches to a high-contrast stylesheet. It changes the background to white, most text to black, and it also gives images a 1px black border.
Her site is the only one that I’ve ever seen this implemented on, and I’ve always thought it was incredibly thoughtful (like most things Veerle designs).
The button can be seen, and used, on this page – just below the title, but before the first paragraph;
http://veerle.duoh.com/design/article/the_best_print_or_web_related_tip
