I've been running Steam on my HTC Wildfire for a few days. It's great and all, but it drains my battery so fast if I leave it running in the background to receive push notifications.
One thing to note, if your phone is set up to automatically receive email from the account you use with Steam without requiring a login, then setting up the Steam app on that phone = BAD IDEA
I signed up and got the invite only a day later. You also get an invite you can pass on to someone.
So far it has been interesting, but you have to make sure that you shut it down when you're done. Advanced Task Killer did not shut it down.
I'll mostly use it to chat with my great Edgegamers friends!
Also, people already in the beta can invite other people to it. PM me (or anyone else you know who's in the beta) and I'm sure someone can get you in if Valve doesn't accept you naturally (I got in in less than a day)