The generated full url is incorrect, but the applied fix  only works for 1.9.1+ ruby version.
As the URL manipulation has nothing to do with the File object, I've replaced it with call to URI.
Only tested on ruby 1.8.7
Generate the correct full URL path using the URI class instead of File.
Thanks for the patch.
Can you release another gem version, so I can use it instead of relying on my fork ?
Version 1.0.1 is now available with your patch: http://rubygems.org/gems/big_sitemap