So I've fixed my CVS/Maven issues. It turns out that you need to specify a CVS repository like:

scm:cvs:pserver:anonymous@cvs.sourceforge.net:/cvsroot:classifier4j

rather than

scm:cvs:pserver:anonymous@cvs.sourceforge.net:/cvsroot/classifier4j

Apparently, this is assumed knowledge or something, or is just a CVSism.

I was basing my setting on the sourceforge documentation:

cvs -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/classifier4j login

cvs -z3 -d:pserver:anonymous@cvs.sourceforge.net:/cvsroot/classifier4j comodulename

The SF documentation is usually
quite good…. Oh well… the joys of different development environments.