Don't make 'readonly' fields less readable

I disagree. People know that they can't change the contents of a greyed out box. Less savvy people may see a box with black text and think that there is something wrong with their computer, keyboard or the web page if they can't click in/edit the contents.

Also consider that if things need to be read, high contrast is better. I think github, as an example, suffers from low contrast colors on many things, the file list being one; it's relatively hard (I need to put more effort into it than I think should be necessary) to see on every LCD screen I've tried it on. Obviously, I can fix this myself using greasemonkey or user stylesheets or something, but it seems to be a growing trend.

Making unusable inputs harder to read is an affordance that makes it easier to scan the page for things that <i>are</i> usable. If your page has a scattershot of usable and unusable controls, the problem is probably more fundamental than how you grey things out.