It is possible that you're being blocked by Google's Andriod Marketplace filters. The Slingplayer Mobile is marked as a paid, copy protected application. So, you could be filtered out due to location, using an unrecognized operating system, or having a rooted phone.
You might want to check Google's Andriod Market Support (http://market.android.com/support/) for further assitance, as there are workarounds available depending on your particular issue.
Thank you for the canned response... If you take the time to read my message, you will see it is NOT a rooted phone. Also, this is an Alltel HTC Hero, a US based phone, on a US based carrier. I did not confirm location however,so I will state that I am in the United States as far as location goes.