The marketplace is indeed telling me that the AllShare App is not available for my device.
Edit: Also, I tried to reinstall it via the web marketplace since I had previously installed it on my Focus, and that was a no-go as well.
Edit 2: I tried to reinstall it on my old Focus as well and it didn't work either. Perhaps Samsung or MS have done something to make the App unavailable, or perhaps it is AT&T.