As I have mentioned in the past, Address Book in Tiger can now print Envelopes in addition to labels. I printed some envelopes recently for sending holiday cards. On one of the envelopes I noticed that it printed my friend's name AND his son's name. I started wondering how this feature worked, so I put in a spouse for one of my female friends and nothing happened. I put in a child for one of my female friends and that worked. I was mystified thinking why would this feature work with children and not spouses? macosxhints.com had this tip the other day and they shed a little more light on it being a US feature, but no answer as to the spouse question. Then I figured it out!
The reason the spouse was being left off is that it doesn't work if Address Book thinks the entry is a female (based on the first name).

So if you have a contact with a male's name, for example:

John Smith and you put in a spouse of "Mary" the envelope will print "John Smith and Mary"

If you put Mary Smith in the spouse field, the envelope will print "John and Mary Smith"

If you put Jack in the child field, the envelope will print "John and Mary Smith and Jack"

If you put Jack Smith in the child field, then the envelope will print "John, Mary and Jack Smith"

However, if your contact is Mary Smith, and you put John Smith in the spouse field, the envelope will print "Mary Smith". NOT, Mary and John Smith.

If you're wondering how to add Spouse and Child fields to a record in Address Book, choose Add Field, Related Names from the Card menu in Address Book!