I guess you could try reaching out to one of the many indie developers on Steam.
Anyhow, what I suspect though, is that Steam isn't going to become some "app store" like the current Android and Apple ones, rife with fart apps and other useless things. I suspect it would stay highly curated.
For example you don't see hundreds of amateur flash games on Steam, everything is cherry picked which makes for a great consumer experience. I know that any game I buy off Steam is going to be of high quality.
This has the added benefit of not having any of those "scammer" apps around. You know, those apps that get purchased by any and all hacked Apple accounts just to get pushed to the top of the app store.
Right at the top of the FAQ page:
http://www.steampowered.com/steamworks/FAQ.php
"At the moment Steam is currently not accepting new game submissions as we transition to our new Steam Greenlight process."
Previously, I believe you had to contact them and talk to a human being to start the submission process.
