Here are some questions that you should make sure you cover:

Who will be there?
How many events do you try and book in a weekend?
What will the D.J. be wearing?
Do you work hourly or in a package?
What are the costs for additional hours?
What type of music do you bring?
Do you have any references?
Do you have a web site?
Do you have a light show? (You can expand quite a bit on this one... does it cost more, how do you set it up, etc.)
How much room do you need?
Have you ever D.J.'ed in where I'll be having my reception?
Will you MC the event?
Do you have backup Equipment?
Are you an active part in the planning process?
Do you offer any help planning?
Do you have an office - (don't be worried about this one to much. Most D.J.'s work when there are dances to be D.J.'ed.... usually Friday and Saturday night.... therefore, many D.J.'s work other full time jobs during the week..)
How do I book your services? Are there contracts? Deposits? etc.
Do you provide Dinner Music? (what is the charge?)

Most importantly... How Long have YOU been in business.. and how long have YOU (or your D.J.'s that will be at the wedding) been D.J.'ing? Don't be afraid to ask for references from several years back!

Also, don't forget to note how organized your D.J. is. Ask for information to be mailed to you. Then find out how long it takes to get there. Leave messages on a machine, see when you get called back. E-mail them with questions, note how long it takes for them to respond. Again, this is the most important day of your life, and for a large part of the day, the D.J. will be putting into place a lot of the plans that you've made.... make sure they're capable of doing just that!