...and finding differences:
To checkout 1.2.2, for example, use:
cvs -q tag WordPress-Ver-1_2_2-20041216
(This is a release tag, a marker for a point in time, not a branch tag.)
Having valid release tags makes it much easier to answer questions about what changed. E.g. in response to this question http://wordpress.org/support/3/18692/
You can use cvs -q diff -uwb -rWordPress-Ver-1_2_1-20041006 -rWordPress-Ver-1_2_2-20041216
to get a unified diff of the changes. Drop the u option to get a normal context diff.
or cvs -q rdiff -s -rWordPress-Ver-1_2_1-20041006 -rWordPress-Ver-1_2_2-20041216 wordpress
will get you a list of changed files and their version numbers.
From Mike :)