Joshua Haberman gives some details here: http://article.gmane.org/gmane.comp.lang.lua.general/62321

Our Lua usage isn't too widespread at the moment; it's really one infrastructure project in particular that uses Lua to allow user-defined functions to run within a tightly controlled container. Lua was the best choice, because of its low overhead, fast execution, and the ability to set limits on execution time.

Unfortunately I cannot be more specific, since the project is not public.