It's true that Apple is hostile (= doesn't release spec, but doesn't actively make the protocol hard to reverse-engineer either, eg: the DRM stuff is limited to DRM content and not part of the protocol proper), but the protocol has been reverse engineered and there are many implementations of AirPlay nowadays.

What is the precedence on which you base your belief that, whatever protocol Valve has come up with to replicate the video stream, it will be open? I'm not that much into gaming, but Steam is a closed-source software that includes mandatory DRM for the games, it doesn't really set a precedent of openness and interoperability.

Couple of reasons. First off, Steam is the only DRM I can think of that actually adds value and makes my life easier instead of the reverse.

Second, keep in mind that they've already announced streaming inside the LAN. Having mobile apps would be a natural extension of that. (Note I didn't say that it would be open, I said they will probably release mobile apps).

That streaming is also completely not a threat to their system.

I'm not fully following you: what should these mobile apps allow you to do? Stream the video of a games running on your phone to the steambox to get to the TV?

Function as another endpoint for the stream. Instead of a TV, you have a tablet or a phone. Onlive already does this.