My results here...

Don's Hello World Longhorn App which is a bit more involved than mine.
His uses code in a CDATA section of the XAML to produce Console.WriteLine( "Hello World" );

Sam Gentile shows us his quick and dirty Longhorn Hello World using cmd line and MSBUILD.
Writing your first Longhorn App
(btw, this works without the Longhorn SDK installed as described in the text -- my VPC environment for the time being has ONLY the Longhorn OS)