I actually just went down to Casting this past Thursday, not to apply for the CP, though, a PT position (which, btw, I got hired on the spot! My first day, which is Traditions, is this Saturday!). I saw lots of people in jeans and T-Shirts, but to be honest, I don't think they think too highly of you maintaining The Disney Look (btw, they give you a booklet explaining exactly what The Disney Look is when you are hired) I had just come from school, and dress code there is professional casual, so I really didn't hafta go home and change or anything. They ask you the typical questions, like "Have you ever been convicted of a crime?" and "Do you have any pending charges against you?" My interviewer (her name was Vicki) threw in a question about the company's rep when it comes to Guest service, and I said something like going that extra mile for the Guest and that really makes the vacation complete for them, etc, and she offered me a position right there.
Regardless of whether or not they say it's okay for you to be in jeans, it's not too good of an idea, since you're trying to make a good first impression. Definitely look presentable, and be very enthusiastic and smiling and make eye contact ALWAYS. When hired, they even give you a card that lists those as guidelines to good Guest service.
Good luck!