brew --cache [formula]

This developer-oriented command lets you ask Homebrew what the cached
filename will be for a brew's tarball.

Library/Homebrew/download_strategy.rb

@@ -53,6 +53,8 @@ class AbstractDownloadStrategy
 end
 
 class CurlDownloadStrategy <AbstractDownloadStrategy
+  attr_reader :tarball_path
+  
   def initialize url, name, version, specs
     super
     if @unique_token

Library/Homebrew/formula.rb

@@ -133,6 +133,10 @@ class Formula
     self.class.path name
   end
 
+  def cached_download
+    @downloader.tarball_path
+  end
+
   attr_reader :url, :version, :homepage, :name, :specs
 
   def bin; prefix+'bin' end

bin/brew

@@ -14,7 +14,14 @@ require 'global'
 
 case ARGV.first
 when '--cache'
-  puts HOMEBREW_CACHE
+  if ARGV.named.empty?
+    puts HOMEBREW_CACHE
+  else
+    require 'formula'
+    ARGV.named.each do |name|
+      puts Formula.factory(name).cached_download rescue '(built from source control)'
+    end
+  end
   exit 0
 when '-h', '--help', '--usage', '-?', 'help', nil
   puts ARGV.usage