A lot of people in my class are from the city. I would say about 10-15 people are classified as rural. I think, as the aim of the course is encourage more graduates to work rurally, that a rural background is beneficial but it certainly doesn't preclude you from getting a spot in the course by any means.
Your written application is very important as it is a key factor in deciding your interview offer - remember there are over 1000 applicants each year and you have to write a pretty stellar application to be considered. Don't regurgitate generic or cheesy answers like "I love teeth" or "I love helping people so I want to be a dentist". They have seen that a million times before and those kinds of answers don't show that you've put effort into your app. Make your application original, and if you really want to get in do above and beyond. Do your research on the course, what it means to be a dental professional and really demonstrate in your application that you've indeed done that research and respond thoughtfully and be insightful in your answer. Don't waffle, get to the point, but be original.
And of course as has been mentioned the higher your GPA the better. I would aim for 6+ (5 being the bare minimum). Don't fret if you don't get there though, there are people who've gotten in with less.
All the best with your app.
[offtopic]Wow, my advice for the app sounded creepily similar to what I got told when I was writing essays for English... [/offtopic]