I’ve published a full howto, including Lighttpd and FastCGI?, at
http://bougyman.com/miscfiles/RailsonDebian.html Broken link
Looks as though it has moved:
http://old.bougyman.com/RailsonDebian.html
You should be able to just pull the \FastCgi/Lighttpd configuration from it, ignoring the Rails install and Debian specifics.
See also:
http://duncandavidson.com/essay/2005/12/railsonlighty