Thank you, Valve! But...

I have a vested interest in Steam (in that I have ~500 games in Steam, ~140 of which run on Linux), and would be quite interested in contributing from time to time if they did decide to open source the client - I've already written some third party tools to work with Steam games (in my junk code on github).

They do have DRM, but I don't see it as being a significant problem for open sourcing the client - it only affects Windows games (not Linux, I don't know about Mac) and works by the server wrapping the game's executable to tie it to the particular Steam installation the user is running on (the wrapped executable does not need to be online to verify the environment is what it expects). If their environment changes or they log in elsewhere the server simply generates a new executable for them to download - no install limits and no always online requirements. The point is, the client has very little to do with the DRM they employ.

Steam has always been designed with the philosophy that users should be able to play the games they own as easily as possible (as opposed to other DRM platforms who's philosophy seems to be that customers are nasty buggers and should be prevented at all costs from using the products they may or may not have paid for if there is even the slightest doubt they don't own them - a subtle but rather significant difference), so their DRM has been designed to keep out of the way as much as possible.

When it has bitten me, is when a publisher requires keys for their games and they underestimated how many copies they were going to sell in a weekend sale (they did resolve it early on the Monday when they could get more keys from the publisher, but there were a lot of people who couldn't play Burnout Paradise that weekend).

One thing I can see as being a concern for them if they did open source the client, is people creating fake/proxy steam libraries that lie about what licenses the user has - a lot of games ship DLC files with the main game (for multiplayer games it is often not a choice since they need the art assets to render DLC that other players are using), and Steam has an API that the game can query to find out which DLC the player actually owns. In an open source client it would be trivial to alter the library to tell the game that the user owns whatever DLC they want. Being closed source doesn't prevent this kind of thing from happening, but it does raise the bar quite a bit.