Feature request (dry run version of cadmin update)

A feature that I (and at least one other) would find useful would be a
--dry-run version of "cadmin update", whether presented as
"cadmin update --dry-run" or "cadmin diff" or some other command invocation.

The command (whatever it's called) figures out what changes would take
place if I did a cadmin update, but doesn't actually make those changes.

Why? Say I have a cron job that silently rsyncs the Red Hat updates (I 
haven't found that magic verbosity level that does "only output when
there is something to download") and I want to know what packages would get
upgraded when I update because I have site-specific packages that could get
overridden. The only way I can come close to doing that now is to write up a
script that stores a list of all the rpms in my directories and diffs them
against the last snapshot--which kind of works, but still doesn't 100% match
what cadmin update will do when I run it.

I guess the alternative to that is to have a "cadmin undo".
/me watches hunter scream/shudder

-- 
"UNIX does not prevent you from doing stupid things because that would also
 prevent you from doing clever things" -- Unknown [by me]