How long have you been in the business photographing weddings?
Don't be falsely impressed with commercial photographers... A wedding is entirely different than a studio shoot.

Do you belong to any trade organizations or associations?
This question can be misleading, but if the answer is yes, then chances are they are a serious professional that cares about the betterment of their profession.

How would you describe your approach and style of photography?

Do you have an example of a complete wedding album I can look at? An overall portfolio of your work?
Viewing a complete album will give you an idea of what sort of shots are included. Viewing a portfolio will give you an idea of what other style the photographer may be proficient in. You may find something you like in a portfolio that normally would not be included in a wedding album.

Who exactly will be photographing my wedding?
You should be seeing samples of this individual's work.

How will this person be dressed?
As amazing as it sounds, there are those who dress completely inappropriately for a wedding! If your photographer or their assistant are wearing red pants, you probably need to look elsewhere. (True story)

What kind of camera and equipment do you use?

Do you have a back-up photographer in case you get sick? Describe his/her qualifications. Do you bring back up equipment? 

How long will you stay at the ceremony and reception? 

How many photographs will you take in all?

Are the proofs and / or negatives included in my package? Do you offer them for purchase?

Do you retouch negatives?

Do you offer parent's albums?

What is your payment policy? 
Deposit amount? Due when? Balance due when? Forms of payment accepted?

Will you have a contract outlining exactly what products and services I will receive?
Never, under any circumstances, should you fail to get a contract in writing.