apt pinning

[Nathan Sprangers]

I followed the directions for keeping a mixed system at 
http://www.ubuntulinux.org/wiki/PinningHowto, however it does not work 
as expected for me.  I'm trying to maintain a warty install with only 
some hoary packages, so when I apt-get upgrade I don't want to be 
upgrading to hoary.  Simply adding APT::Default-Release "warty"; didn't 
do this for me.  When I run apt-get upgrade it wants to upgrade 
everything.  What I does work for me, though, is putting this in 
/etc/apt/preferences:

Package: *
Pin: release a=hoary
Pin-Priority: 50

(I got that from man apt_preferences, just changed unstable to hoary)
Why doesn't the wiki way work for me?
~Nathan