I just configured my ubuntu based XBMC box to be an airplay recipient using PascalW's Airplayer. Unfortunately, it works with video and picture streaming, but it is easy to setup (just follow the instructions in the package), and just works.
Next step: getting shairport to work. Update: It's simpler than I thought. I now have shairport working, now to daemonize: something like this should work.
Jonathan Keane
blog
Musings and discoveries while navigating life. Mostly python, perl, sql, R, linux, os x, linguistics, cooking, bread, and food.