Valve may very well produce one of the near future's most popular non-mobile, consumer, Linux distributions. SteamOS will be marketed for gaming PCs (some very compelling ones at that) starting next year. CES will definitely be interesting. With such a popular distribution, and as an existing member of the Khronos Group, it makes sense for Valve to join the Linux Foundation… and they just did.
It is still unknown to what extent Valve joined Linux (members are classified by level of contribution from Platinum to Silver) and we likely will not know until their list is updated. While they probably will not be hanging out with Intel and others in the platinum category, Silver is not the most noteworthy of statuses… alongside Barnes and Noble (likely because of the Nook) and Twitter.
Another addition is the HSA Foundation. AMD is already a Gold member (y'know… HSA's faja) and ARM is Silver so I cannot see HSA being much more than that. Still, Linux will be an important focus for the heterogeneous computing architectures to endorse: both in terms of back-end server optimization and customer-facing devices.
Of course I am not belittling any contribution. Still, there is that desire to see Valve lead the pack. Ultimately, though, it is not the size of the badge: it is how you wear it.