$HTTP["host"] =~ "^www\.(.*)" { url.redirect = ( "^/(.*)" => "http://%1/$1" ) }
Add following configuration directive if you want to redirect domain.com to www.domain.com (force www)
$HTTP["host"] =~ "^domain\.com$" { url.redirect = ( "^/(.*)" => "http://www.domain.com/$1" ) }
Credit goes to Vivek Gite from Nixcraft (http://www.cyberciti.biz/tips/lighttpd-redirect-www-domaincom-to-domain-com.html)