Just tried with -N, like this:
export CVSROOT=":pserver:mailto:anonymous@openacs.org:/cvsroot"
cvs login
(enter)
cvs -z3 co -r oacs-4-6 -d oacs-dotlrn -N acs-core
and it pulled everything in... But: it created oacs-dotlrn/openacs-4 and pulled the stuff you wanted to be in oacs-dotlrn/ into the dir oacs-dotlrn/openacs-4.
Perhaps the man page ("man cvs") will have an answer as to why... let me know if so :)
-Jim