Make sure you have mod_redirect in server.modules, then add this to your lighttpd.conf:

$HTTP["host"] =~ "^domain\.com$" {
  url.redirect = (
    "^/(.*)" => "http://www.domain.com/$1",
    ""       => "http://www.domain.com/"
  )
}

I wasn't able to come up with a way to do it in a single rule. It seems that only "" (not even "(.*)") will match the root request.