I have a WDTV Live, and it comes up in the list of devices on the YouTube app on my Android phone.

You can (and many tv and STB manufacturers already have) implement the DIAL protocol[1] to make your device show up in the list of targets to cast to.

[1] http://www.dial-multiscreen.org/

I'm not entirely sure, but I think so, I remember someone saying that it was already possible with some models of an LG tvs. I think it's called the DIAL protocol.

Other manufacturers can add the software to their device, in cooperation with google.