Who cares about the starving orphans so long as I get cheap games? Re: Valve's cut. Developers aren't allowed to say but the 'hypotheticals' that have been thrown out seem to be around the 25-30% range. That's for games sold through Steam. No idea if they get a cut from third-party sales.
It's 30% for most, though I assume major players like Activision, Ubi, etc. negotiate lower rates.
Steam codes can be generated for free by developers so 3rd party sales don't directly profit Valve (for now).
Once Steam becomes a general purpose CDN, things will change. Valve will probably (hopefully?) charge a lower rate to post & serve games ("content") but they won't have to do squat except maintain the actual network. All the posting of games, patches, news, etc. will be the dev's job. Most likely, *everyone* will be able to put whatever crap they want on Steam - there may not even be an actual "curated" Steam store any more. We'll see.