The deadlift seems to put a lot of pressure on the spinal vertebrae. Is it safe to deadlift without a belt or does correct form completely eliminate the need for one?
Using a belt is like training wheels for bicycles. Sure, they will help you increase your weight more quickly then without one, but with proper form and care for your body, you can maintain your skills better.
Deadlifting is a exercise that uses many stabilizing muscles, and if you do not have all of them working together, that's an easy road to injury.
Source: Got a hernia in college from forgetting to wear a belt during a workout. The doctor said that I was using the belt as a crutch so I didn't work my core muscles enough. Once I recovered, I found a trainer and completely reworked my form to focus more on helping my core support the rest of my body. I still cannot lift as much as I did during college, but I've improved in more areas and I've limited my chance for injury.
More sources: The article by Paul Chek is particularly informative.
As I understand it, belts are used in powerlifting to provide something for the abs to push against, which increases intra-abdominal pressure and locks the trunk in place during the lift. This helps prevent movement of the lumbar spine during the exercise.
As a beginner lifter (300# deadlift), I do not find it necessary. Some people consider it helpful when lifting heavy. Others consider it to be a crutch.
Also, this question is similar but not quite the same, I think.