Just as an extra comment: if they REALLY love open source, and it is only, as you said, because Steam wouldn't succeed being open source that it is closed, why they don't open the Gold Source engine? It is not actively being sold (as it is obsolete), the source code is obviously not lost, as they ported it to Linux just months ago, and is unrelated to Steam (as the service that gets them most of the money). They would be making a nice gift to free and open source software, and it doesn't represent any loss to them.
No announcement yet.
Valve Joins The Linux Foundation