There has been some confusing and conflicting information about whether visitors are allowed to wear jeans on tours to the DMZ. Each tour or travel website seems to have its own rules about what you should or should not wear.
You have nothing to worry about wearing jeans as long as they are clean and not faded, ripped, or torn in any way. On a recent tour taken in January 2012, almost everyone was wearing jeans without any issues.
Jean shorts, or any shorts, are not allowed.