I've just done the following with cvs (version 1.11nb2).

cvs checkout -rnetbsd-1-5 src
cd src && cvs update -dP

On the update, I have gained one or two directories and files from
the trunk (e.g. usr.sbin/installboot/ and its contents (which will stop
1.5.* from doing a "make release" of itself properly)).

Is this the correct behaviour of cvs? (Admittedly, I did not use a -P 
to "cvs checkout".)

best wishes,
Chris