Funny, I've had this "app" for quite some time.
I had a bookmark of the Steam store saved on the home screen.
From the looks of this app, it does the same exact thing, except with access to your friends list (which I already get that from Raptr and it's mobile app).
I've always wished that if Valve actually did release something like this, it would have some sort of distribution service to buy and play certain games on my iPod, but I doubt Apple's draconian policy would allow such a thing.
EDIT: Oh, and it turns out this app is in beta. You can't simply just log in and go, you have to open up the Steam client on your PC/Mac and change your beta participation settings. Yay.
EDIT AGAIN: Upon further testing, it seems that it's in closed beta, which means you need
a beta key to even try the app out.
Or you could save the trouble and just go to the Steam store on Safari and hit "Save to home screen" and cut out all the bullshit.