Like an idiot I accidentally overwrote a file for this blog when uploading some other stuff; accidents happen. Anyway, I have backups (you do have backups right?) and I needed to grab a single file out of the latest tar. Fortunately this is really straight forward and rather than unpacking the entire backup of all sites + dbs just to get one file you can use something like this:
tar -xf backup.1194753607.tar --wildcards *blah/index.php
As the manual states the wildcards flag enables globbing. Here's a quick hack to flatten the output. Instead of creating the directory structure the -O flag is used to send the output to stdout and then redirect that output into the file index.php.
tar -xf backup.1194753607.tar -O --wildcards *blah/index.php > index.php