GPL cannot force somebody to release their source code, as long as they are not supposed to have the binary (for example, internal studio - you can use binary tools compiled ith GPL, and be happy knowing that that stuff does not go outside of the company).
You are mistaken if you believe that you can distribute an application built using GPL code and not release the source code (and all derived code) as well. In the event that you were confused by my wording, check again. I have attempted to clarify.
That's not what your original post said, and even now that you've edited it, it's still wrong. There is no requirement that Valve release source code for an internal development build, and just because GPL'd code was at one point used in Steam wouldn't prevent them from distributing it without source in the future.
The GPL isn't some magical infective uber-license; it's just plain copyright.
Right, it can't force them to release an internal build. But that's not what this is, it's an external beta. At this point they could still back out from releasing the source code if they wished, but not if they had used the GPL. If they had used a GPL program, they would at this point have to sacrifice a great deal of work to avoid releasing the source. That is my point. The GPL can force you to plan for things that you have no way of knowing about (For example, are we going to have to modify this program to couple with our internal APIs at a level that we don't want documented? Is the source code going to be ugly to an extent that it would be embarrassing to us to release it? etc.).
The GPL can't force Valve to release source even if they had used GPL'ed code for an external release. However, a court could force Valve to stop distributing the infringing binary, and to pay damages to the owners of the GPL'ed code.
It is possible that Valve would choose to release source for the steam client after negotiation with the owners of the CPL'ed code instead, but that would require agreement from both parties. And it is actually not totally unlikely that Valve would seek such a solution, the steam client represents a very little part of the total value of steam, the main value is in the content of their rights database, and in their reputation and connection with gamers and publishers.
