Lighttpd config (sections):

server.username      = "lighttpd"
server.groupname     = "lighttpd"
server.chroot      = "/home/www"
server.document-root = "/public_html/"

server.modules += ("mod_fastcgi")
fastcgi.server = ( ".php" =>
                            ( "localhost" =>
                                   (
                                                "socket"        => "/tmp/php-fastcgi.socket",
                                                "bin-path"      => "/bin/php-cgi",
                                                "bin-environment" => (
                                                        "PHP_FCGI_MAX_REQUESTS" => "10000"
                                                        ),
                                                "bin-copy-environment" => (
                                                        "SHELL")
                                    )
                            )
           )



Error:
2006-07-07 12:03:08: (mod_fastcgi.c.1022) execve failed for: /bin/php-cgi No such file or directory
2006-07-07 12:03:08: (mod_fastcgi.c.1048) the fastcgi-backend /bin/php-cgi failed to start:
2006-07-07 12:03:08: (mod_fastcgi.c.1052) child exited with status 2 /bin/php-cgi
2006-07-07 12:03:08: (mod_fastcgi.c.1055) if you try do run PHP as FastCGI backend make sure you use the FastCGI enabled version.
You can find out if it is the right one by executing 'php -v' and it should display '(cgi-fcgi)' in the output, NOT (cgi) NOR (cli)
For more information check http://www.lighttpd.net/documentation/fastcgi.html#preparing-php-as-a-fastcgi-program
2006-07-07 12:03:08: (mod_fastcgi.c.1060) If this is PHP on Gentoo add fastcgi to the USE flags
2006-07-07 12:03:08: (mod_fastcgi.c.1356) [ERROR]: spawning fcgi failed.
2006-07-07 12:03:08: (server.c.834) Configuration of plugins failed. Going down.


Directory structure of /home/www/:
./bin
./bin/php-cgi
./etc
./etc/php
./etc/php/cgi-php5
./etc/php/cgi-php5/php.ini
./lib
./lib/tls
./lib/tls/libm.so.6
./lib/tls/libc.so.6
./lib/libreadline.so.5
./lib/libresolv.so.2
./lib/libz.so.1
./lib/libnsl.so.1
./lib/libhistory.so.5
./lib/libbz2.so.1
./lib/libncurses.so.5
./lib/libgcc_s.so.1
./lib/libgpm.so.1
./lib/libcrypt.so.1
./lib/libdl.so.2
./tmp
./tmp/php-fastcgi.socket-0
./usr
./usr/lib
./usr/lib/gcc
./usr/lib/gcc/x86_64-pc-linux-gnu
./usr/lib/gcc/x86_64-pc-linux-gnu/3.4.5
./usr/lib/gcc/x86_64-pc-linux-gnu/3.4.5/libstdc++.so.6
./usr/lib/libssl.so.0.9.7
./usr/lib/libjpeg.so.62
./usr/lib/libmcrypt.so.4
./usr/lib/libt1.so.5
./usr/lib/libltdl.so.3
./usr/lib/libcrypto.so.0.9.7
./usr/lib/libxml2.so.2
./usr/lib/libpng.so.3
./usr/lib/libaspell.so.15
./usr/lib/libpspell.so.15
./usr/lib/libfreetype.so.6
./usr/lib/libcurl.so.3
./usr/lib/libpanel.so.5
./usr/lib/libmm.so.13
./usr/lib64
./usr/lib64/mysql
./usr/lib64/mysql/libmysqlclient.so.15
./logs
./public_html
./public_html/index.php

(all are chowned to lighttpd:lighttpd including /home/www/)

try using this:
url.rewrite                = ( 
                                "^(/.*)$" => "/bin/php.cgi$1",
                             )