Android already has a game store. It doesn't make any business sense for Valve to try to compete with Google Play Store.
About Portal: It has to be profitable enough to justify the huge cost of porting the game to Android. The same for any other game.
I'm excited for the eventual Steam Apk Store and Steam Games for Android.
Maybe Valve will distribute Steam Android devices, but that seems silly.
If Valve wants to get mobile, they need to hope for the success of Ubuntu Phone / Plasma Active / Tizen. Even Ubuntu Phone is probably going to block in the Canonical app store competing markets.
Here's a reddit link that goes to the presentation slides but also has some comments from one of the presenters:
http://www.reddit.com/r/linux_gaming/comments/1bnaoh/valves_...