Linux developers doesn't necessarily imply there'll be Steam for Linux Desktop™. It may be that Valve is looking to finally build their Steam Console and base it on Linux.
http://www.phoronix.com/scan.php?page=article&item=valve...
It is of course possible that they are planning a console and that this is why they are focusing their efforts.
If they do develop a Linux based console then hopefully it will still make sense for them to support "proper" Linux as part of that.
Valve can not really enter the mobile marketplace in a meaningful way. Both Microsoft and Apple will likely try to put the squeeze on Valve with their own respective OS marketplaces. (I think neither will successfully be able reproduce Steam's high quality user experience and extras.)
I think we are about to see a divergence in desktop/HTPC/semi-mobile platforms.
For example, ChromeBook/Box , Raspberry Pi , Android , Ubuntu Devices and I'm sure many others as time goes on.
it seems reasonable that people will want to run games on these platforms beyond what is currently possible just with JS/webGL.
Now think about it, what do all of these plaftorms have in common?
Another point to bear in mind , is that one of the significant advantages Steam has over OS specific marketplaces is that it allows you to sync the same games over different devices.
For example it would be nice to be able to buy a game for your Windows PC and then be able to also run it on your HTPC or Tablet device which may well be running non Windows OS.
Microsoft is playing a longer game than most people can see. So is Valve.
I've had an employee (friend) verify these facts to me.