apt-get "pinning"

[Tom Hoover <tom-key-debian.a101e4@hisword.net>]

I'm trying to use the new "pin" feature to track woody, but allow
installation of individual packages from sid.  My /etc/apt/preferences
files is as follows:

Package: *
Pin: release a=woody
Pin-Priority: 700

Package: *
Pin: release a=sid
Pin-Priority: 70

"apt-get install xxx" & "apt-get -t sid install xxx" work as expected,
but "apt-get upgrade" tries to upgrade over 300 packages on my system to
sid.  I can remove all references to sid from my sources.list, perform
an "apt-get update; apt-get upgrade", and it works as expected.  Am I
misunderstanding the use of the pins?  Do they only work with "apt-get
install", and not "apt-get upgrade"?  Or, do I have something
misconfigured?

Thanks!