Some days ago I stumbled upon an old entry of Jan in lighty's life, called "X-Sendfile". There he explains how to speed up the delivery of (large) files with lighttpd instead of PHP (YES, lighttpd is very fast - for one customer we created an ImageServer with pure lighty that replaced a 4-server-cluster with Apache and now has 1 server with lighttpd (which is boring around at low load). The box makes 180 Mio. requests per month).
He even gives an example of the functionality, showing how a combination of an entry in the web server's config coupled with a simple PHP script can easily send out a large file to anyone nice and zippy.