searching for what package includes a given file

[Benjamin Rutt <rutt+news@cis.ohio-state.edu>]

I just successfully used "Search the contents of packages" from
http://www.debian.org/distrib/packages to determine what package in
"stable" provides GL/glx.h.

Is there a way to do that search via command-line tools without
resorting to the web page?  I know that dpkg -S <expression> will do
that for packages already installed on your system, but doesn't help
for those that are not.
-- 
Benjamin