So if you're using virtualenv 1.7 and are wondering why you can no loger see your global Python site-packages directory it's because the default behavior has changed.
Now the default install for virtualenv is to hide site packages; equivalent to passing the old --no-site-packages command:
virtualenv --no-site-packages <env>
Since this option is now default it's no longer needed, and the new command to see the global site packages is --system-site-packages:
vitualenv --system-site-packages <env>
Hopefully this saves you some confusion as it's not clear what's happening when your new install now suddenly is breaking.