Hi --
I've written a little plugin, partly just for fun (though there's no reason it couldn't be useful), called add_downs, which lets you do this:
rake db:migrate:add_downs
and end up with your migrations with their down methods written automatically.
The way it works is that it overrides create_table, add_column, etc., so that they print out their opposites, and then it executes your up methods. It doesn't connect to the database, so it can't reverse certain operations involving column type.
Overall it's mostly for simple-ish cases, but could be handy. You can find it at: http://www.risleydale.net/svn/plugins/add_downs
David