We've had a chance to play with the app, and it certainly gets the job done -- we just wish it worked a little better. Even on WiFi, control feels laggy, and there's no native guide (though there's an icon to quickly pull up your set-top box's guide). When you're in letterbox mode, you've got black bars around all four sides of the image, meaning that the app isn't taking full advantage of the phone's display; we're hopeful this'll be fixed in a future release, though. Happily, we think Sling really nailed the app's UI -- the on-screen translucent controls are pretty cool and generally work well without obscuring the view. Look to grab your own copy some time between 12:01AM and 6:01AM EDT tomorrow morning, and in the meantime, check out our gallery.
Update: The app's now available, so get ready to part with $30 (and to not part with your 3G bandwidth -- you certainly won't need it here). Thanks, everyone!
SlingPlayer Mobile for iPhone hands-on