The package is in squeeze/sid, just install libapache2-mod-xsendfile after adding either repo to your /etc/apt/sources.list.
I dont know why it wasnt included before. Its not in stable atleast, that much i know.
I dont have the rank to edit other users posts but here goes Sam.
If you want to use dpkg -i to install the package you will have to download the individual package, choose architecture and download.
dpkg -i libapache2-mod-xsendfile_0.9-2_i386.deb
I suggest adding squeeze or sid to /etc/apt/sources.list and then use apt (will download, install and satisfy any dependencies for the package):
apt-get install libapache2-mod-xsendfile
If you want to use apt-pinning use something like this in your /etc/apt/preferences
Pin: release a=stable
Pin: release a=testing
Pin: release a=unstable
This will cause stable to be of higher priority than testing(squeeze) and unstable(sid) and thus use stable as default when downloading packages with the apt tools.