The 2009 version of MLB.com At Bat, available on April 2nd, will support Gameday Audio. It is not known if a separate subscription ($14.95) will be required. There are no blackout restrictions on audio.
That is fantastic news! I'm guessing the app will probably only allow you to listen to audio if you're a full subscriber to the MLB.com radio service (I usually do this every year) but to be able to access the feeds from the iPhone is awesome.