75
I have version 2.0.7 installed on Ubuntu and it defaulted to /var/lib/mongodb/ and that is also what was placed into my /etc/mongodb.conf file.
I depends on the version and the distro.
For example the default download pre-2.2 from the MongoDB site uses: /data/db but the Ubuntu install at one point used to use: var/lib/mongodb.
I think these have been standardised now so that 2.2+ will only use data/db whether it comes from direct download on the site or from the repos.