WMP unable to connect to WMS 2008; WMS returning 503 Service Unavailable

I’ve talked to a number of people that are having problems streaming from Windows Media Services 2008 on Windows Server 2008 when the server is behind a NAT firewall, some proxies, or load balancers.  Clients on the internal network work just fine.  In a network trace you’ll see at WMS is returning a 503 Service Unavailable…

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