This is an informational thread, which I was inspired to post because I've noticed Eric Meyer does the same thing as I do with stylesheet media:
I don't know if his reasons are the same as mine, but here are mine, in case this information is useful to anyone else:
Never use media="all" - because it's evil - how can you possibly know that your CSS is universally suitable? Do you have a "handheld" media device to test on? How about "aural" or "braille"?
So don't apply CSS to a media for which you don't have a test device - in most cases that means "screen" and possibly "print".
So about "projection"? Well - Opera 7 in fullscreen mode uses "projection" media CSS, not "screen" - the point of that is so that the browser can be used to do slideshows and other kinds of presentation - the sort of thing one might use PowerPoint for - except of course that Opera is free, can be easily carried around on a USB drive or ZIP disk, and lets you write your presentations in HTML and CSS - no proprietary data formats or having to buy MS Office.
But from the point of view of most websites, that's a gotcha. What if someone's visiting your website in Opera in fullscreen mode, just because they want to - if all your CSS is "screen" then the site will be plain - no styling at all.
Hence the dual-media declaration: meda="screen,projection"