I do believe that Disney has a policy on attire to some degree...I remember as a child, my older brother had a shirt that proclaimed "I'm tired of busting my A**" with a picture of a cowboy with a big hammer in his hand and a broken up donkey on it....they made my parents purchase a different shirt for him to get into the MK...now that was back in 1977 though...
We are usually at WDW in May or June and have definetly noticed some instances of poor judgement in attire that have gone unchecked, especially in the past 2-3 years....we have witnessed many teenage girls wearing their swimsuits in the parks; not just the tops with shorts, but, the entire swinsuit...although I was not personally offended, I do find this kinda inappropriate...
There are also many "foreign" guest who come from nations were it is perfectly acceptable to wear somewhat skimpy attire...if you go in the summer, you will see them...
we've been during "gay" days on several occasions and although we witnessed some fairly "outlandish" attire; I can't ever recall seeing anyone with offensive attire during this time.
I think the worst I've seen was a couple of years ago at BB: A male in his late teens/early 20's with a shirt on that said: "F*** milk, got pot?" I found this to be unacceptable and did tell a CM...don't know if they did anything about it or not...