The details need to be straightened out prior to your arrival, and you should mention your intent to house offsite during your interview, so things are handled appropriately (the right paperwork is passed along to you, etc). I wouldn't necessarily say it happens all the time and it's easy to get out of housing (it is considered to be a large part of the overall program experience), but it's certainly possible.
After you've arrive, your chances are severely diminished. After becoming a legal resident of Florida and obtaining outside housing (and a lease in my own name for over a month), I was not released from housing for the last month or two of my program in '09. I ate the rent deductions out of my paycheck each week, and technically had two residences: my apartment at Chatham and the apartment I was actually living in. They don't force you to reside in their apartments, but after arrival, you have signed a contract that does stipulate that deductions for rent are valid for the duration of your program - and provisions for exit are not outlined (they're at the sole discretion of Disney Housing's Yield Management) After arrival, the only reasons they generally consider is family having property you can reside in or career advancement (a new position within Disney pending).
