Most EULA's you agree to - whether on Steam or elsewhere for games (and often, for software in general) - do, more or less, spell it out that you do not own the product; you are paying for a "license" to "use" the product, more or less.
By being on Steam, you have already basically agreed to Steam's own EULA known as the Steam Subscriber Agreement (SSA):
http://store.steampowered.com/subscriber_agreement/
Basically, you "subscribe" to a service here (Steam). So, more or less, whatever you have on Steam - you have rented a product...for the time being. You really don't technically own anything here.
Keys get revoked, you lose your license. That's DRM, if I've ever heard of it.
Section 10C of the SSA talks about Revoking. Like most legalese - it's often vague and there's tons of nonsense language thrown it - yes, just like your typical MysterD post, more or less - so they hope you just skip the important stuff, agree to it blindly, and move right along anyways.
Basically, what's stated is this in SSA 10C: Valve can revoke anything at anytime. Purposely vague about what publishers and dev's have for power there, as it's not really spelled out at all. So, I'd guess any of this could happen: if Valve agrees to a request from either the dev, publisher, or both for revoking - well, then history will be made. [shrug]
As much as I love Steam for its great features that put everyone else trying to cobble together some kind of so-called Gaming-Service Program to shame by a mile and whatnot - yeah, at least with DRM-FREE versions, as long as I can keep my files stored somewhere, my game is still mine. It's just a matter of getting it running, if the game gets old & whatnot.