Earlier quoted context omitted.
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.
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.