x-sendfile

Just pushed it to github. Here's the module for Drupal 6. Up and coming is getting the module hosted on d.o and providing a Drupal 7 version.

I've done a small patch, which makes fast private file transfers through X-Sendfile possible.

Instead of changing UI as the old one, this one just provides the possibility to specify a handler for file transferring:
http://drupal.org/node/74472#comment-323445

fago

For those looking to have Drupal serve private files efficiently, I've posted a simple core patch that adds X-Sendfile support:
http://drupal.org/node/74472