Andy, excellent article. I prefer to use simple text encoding schemes that are human readable for things like state. An example of alternative to your numeric encoding is:

U = Unverified

V = Verified

F = Unverified (Follow up sent)

X = Expired (Can be deleted)

The advantage to this is that users who are creating reports on the information will come to understand the encoding scheme easier and won't need to join in another table to get the description. Even when there are just a couple encodings on a table, adding the extra join can be a hassle.

Joe's article on transition constraints is also very good. Here is the link for everyone: http://www.dbazine.com/ofinterest/oi-articles/celko35/view.