In
http://www.flyertalk.com/forum/Forum71/HTML/014729.html
there is at least one person who was told that they couldn't use these certificates if they had previously used a free-miles certificate. (There were two of them back in 2001.) I haven't seen anyone post contrary to this. People have posted that they've been able to use the starter certificate (once per account) if they have previously gotten miles for cereal.
It may or may not be part of the confusion, but the coupons and cereal seem to be posting separately.