They could do something for Android, but iOS and WP7 would never allow a 3rd party "store" application.
They do have a Steam iOS application, but it's an adjunct to the PC Steam application.
I wouldn't mind for them to create cool stuff for iOS (since I own one), but Valve's games don't lend themselves well to portable versions. I haven't seen a portable first-person game done well on any platform (even NDS and PSP). They'd be better off creating new IP or a different genre for mobile (like ID's mobile games - the on-rails shooters aren't bad, and the RPG games are great fun).
> I haven't seen a portable first-person game done well on any platform
Not 1st person but close enough: try Dead Space on iOS. Mass Effect Infiltrator is close behind (and recently got a nice update regarding free aiming). Both nailed touch input for this class of games, avoiding the "shoehorning an on-screen gamepad emulation" kludge entirely (like e.g Max Payne and GTA games, which I did not buy, because the show-me-a-gamepad-on-screen is a dead giveaway of a milking-the-cow release).
As for PSP, it lacked a second stick. Their decision of a single stick design was braindead, especially given the PS/PS2 experience Sony had.
Interesting that you say no one has managed to crack first person on a portable platform. Maybe Michael Abrash can bring some of that game changing "Quake" style magic to mobile (although I guess Carmack is a pretty big missing ingredient).
It's not a technological problem, it's a hardware problem (at least from my non-visionary point of view). Twin thumbsticks are required. Anything else and it's a recipe for frustration.
Metroid Hunters on NDS was probably the best I've personally seen, but it's the best of a poor breed.
The Vita is the first mainstream handheld with twin thumbsticks, but IMO Sony has an extremely hard uphill battle with their device. Portable gaming is moving away from handheld consoles. I would love to see it happen, but I personally doubt we'll see a Quake-level success on the Vita.
They do have a Steam iOS application, but it's an adjunct to the PC Steam application.
I wouldn't mind for them to create cool stuff for iOS (since I own one), but Valve's games don't lend themselves well to portable versions. I haven't seen a portable first-person game done well on any platform (even NDS and PSP). They'd be better off creating new IP or a different genre for mobile (like ID's mobile games - the on-rails shooters aren't bad, and the RPG games are great fun).