Back when I was planning my wedding, I did a lot of lurking on alt.wedding and similar newsgroups. The topic of how to address someone on an invitation came up a lot.
Generally, the word "and" indicates marriage.
If she's a doctor and he isn't, it's:
Dr. Mary Smith and Mr. John Smith
(If a long last name is involved, you can get away with Dr. Mary and Mr. John Smithxxxxxxxxxxx)
If he's a doctor and she isn't, you can use:
Dr. and Mrs. John Smith (traditional)
Dr. and Mrs. John and Mary Smith (less traditional)
If both are doctors:
Drs. Mary and John Smith (as in Wine&Blue's example)
If she kept her maiden name:
Ms. Mary Jones and Mr. John Smith
If she changed her name:
Mr. and Mrs. John Smith (traditional)
Mr. and Mrs. John and Mary Smith (less traditional - and what I prefer

)
If the couple is living together but not married (whether engaged or not):
Ms. (or Miss) Mary Jones
Mr. John Smith
(separate lines, no "and")
If a couple is engaged but not living together, each member of the couple would get a separate invitation.
Traditionally, a widow is Mrs. John Smith, and a divorced woman is Mrs. Mary Smith. So you only get your first name back if you get divorced.
And just to throw something else into the name mix, both my parents are doctors, but my mother earned her degrees and did her internship and residency before she got married - and she and my dad worked at the same hospital. So, professionally, she went by her maiden name, but she used my dad's name on all her legal documents. I remember that she had to double-endorse her paychecks b/c they were made out to her in her maiden name and her bank account was in her married name. Socially, though, she's Mrs. Hisname.