http://disneyland.disney.go.com/faq/general/
"What is the best way to dress for a day at the parks? Is there any clothing that isn't permissible?
The parks are a casual, family-oriented environment. We suggest you dress comfortably, wear good walking shoes and check the local weather report before you leave for the parks.
Ensuring that the parks are family friendly is an important part of the Disney experience. In that spirit, we ask you to use your discretion and common sense.
Attire that is not appropriate for the theme parks (and which may result in refusal of admittance) includes but is not limited to:
Adult costumes or clothing that can be viewed as representative of an actual Disney character
Masks (unless you are dressing up for a particular event)
Clothing with objectionable material, including obscene language or graphics
Excessively torn clothing
Clothing which, by nature, exposes excessive portions of the skin that may be viewed as inappropriate for a family environment
Tattoos that could be considered objectionable, such as with obscene language or graphics
A Guest is allowed into the parks if her or his hair (or make-up, if applicable) has been made to resemble a Disney Princess or character (for example, after a Bibbidi Bobbidi Boutique makeover) provided the Guest is not also wearing a costume or clothed to look like the character."
The website specifies "adult" but does not say whether they mean actual adult or "Disney adult" which may lead to the conflicting viewpoints.