Here are a couple of methods to determine which version of Ubuntu you are running:

The contents of file:///usr/share/ubuntu-artwork/home/index.html has the "Welcome to " information. In my case that is "Welcome to Xubuntu 7.04!".

The version number equates to what year and month the version was released i.e. 7.04 is April of 2007

cat /etc/apt/sources.list | grep -v ^#
will give you what named version you are running. e.g. the output from this command will show you something like the following the name after the url which in this case is "gutsy" is the ubuntu named version. It could be feisty or something else.

deb http://au.archive.ubuntu.com/ubuntu/ gutsy main restricted

This method is probably the best distribution agnostic way of disovering the version because any distribution that is "Linux Standard Base" compliant should have this file.

cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=7.10
DISTRIB_CODENAME=gutsy
DISTRIB_DESCRIPTION="Ubuntu gutsy (development branch)"
or

lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu gutsy (development branch)
Release: 7.10
Codename: gutsy

cat /etc/issue
Ubuntu gutsy (development branch) \n \l