Here are some key resources for builing a repository. Hopefully I'll get a tutorial up soon.
http://forum.boxee.tv/showthread.php?t=16040 - Check out my repo with test apps in it for you to play with
http://www.fuzzthed.com/rss_link_builder.php - Flash/Silverlight Link Builder
http://www.fuzzthed.com/pipestutorial.php - A Yahoo Pipes to Boxee Tutorial
http://www.fuzzthed.com/rsstutorial.php - My RSS app and repository Tutorial
http://developer.boxee.tv/application-repository/ - Boxee's Official Documention.
http://www.boxxxee.com/2009/05/creating-your-own-boxee-repository/ - Boxxxee.com Tutorial