I did read somewhere that you can be asked to leave for bad language on t-shirts and "questionable" clothing, but I would think there's a not shirt, no shoes, no service policy - if not, there should be!
Hmm. I could have sworn I saw somewhere (maybe in a park map?), that you had to wear shoes and shirts in the parks, and I know that they've talked to people in Downtown Disney (especially at the House of Blues) about wearing "gang attire". But I couldn't find a dress code posted anywhere. Hmm. Just bad taste overall, then.
Last summer we noticed a guy lying on one of the benches between the castle and Alice without a shirt on. We were just passing by, but I'm sure eventually a CM would have come by and said something. Even though they don't have the dress code they used to have, I'm pretty sure you have to wear a shirt.
A few years back we were at the park in July when it was very hot, two guys in the early 20's had their shirts off and a cast member told them that they had to put their shirts back on as it was Disneyland policy.
I'm pretty sure you can't walk around shirtless. Many other parks make that rule implicitly known if they have a waterpark area within the main park. "Shirts and shoes must be worn beyond this point" and such. I don't know why this guy wasn't asked to put his shirt on, however.
Warning: Portions of this attraction are intense and may not be appropriate for younger children.
I remember once at Rivers of America, eons ago, seeing them approach a young man at WDW and request he put his short back on. He did, but I also remember it was during the morning--so it wasn't even that warm yet. But you know some guys...didn't Jimmy Kimmel shoot "Mr. Shirtless Guy?" Probably needs to.