One of the more interesting aspects of the business meeting dress code debate is that typically wherever that meeting is held can often lead to seeing fellow attendees in very casual (or no) clothes anyway.
For many years our national association held conferences in Hawaii, and that led to the "opportunity" to see not only our own leadership in bathing suits, but everyone else's leaders (and our colleagues) in bathing suits too. But even if it's not Hawaii or Miami, many hotels now have hot tubs, fitness areas with swimming pools, or may be near water. Water equals bathing suits. And fitness centers can mean gym shorts and tops that look like (and are) sports bras. And those who golf or run are going to wear shorts - in the elevator, in the lobby, etc.
Then there's spas that are inside business hotels/resorts. While checking in with two colleagues recently the spa attendant told us the thermal water area (or whatever it's called -- really warm pool with a waterfall) was "clothing optional." I actually never thought about having a business meeting turn "nakation" (a new term for clothing optional vacations) but we were given that choice.
Some spas have co-ed waiting areas, and I once waited around with a CEO of our national association (a man) with both of us in robes sitting on some giant lounge bed along with the other treatment people. We had spent the prior 8 hours together at a conference table in professional dress.
1. Most retreats and off-site business meetings really are casual these days (shorts, khakis, etc.) - ask the organizer to tell everyone that;
2. Conference attendees WANT to be casual, but sometimes organizers tell them they have to be business casual or business dress. Not sure why it matters to be dressed up if just sitting and listening most of the day. We find better conference "moods" when dress-down;
3. Certain cities - for example, always DC and sometimes Chicago, are just more business attire driven than other cities. If meeting in DC you really can run into a Member of Congress so it's just respectful of position to dress accordingly (yes, I ran into multiple Members of Congress -- who know me -- while wearing loud madras shorts on a flight to DC so don't do that anymore). Many Florida and California cities are naturally casual; as are some industries. So it would be more natural to wear running shorts to an association of runners than to an association of attorneys;
4. Often with in-area meetings that are only part of a day (versus full day) it's most appropriate to wear business attire because members are still working before or after the association meeting. I believe it's respectful to dress in whatever professional manner the members often dress to meet with their clients - as that's what they'll likely wear to your meeting; (anyone who genuinely doesn't know what "business attire" is -- check what your members wear when they're working);
5. Unless actually in a bathing suit, professional clothes should not show the same amount of skin (some people really haven't figured this out?) -- low cut clothing on professional women in professional settings is often considered a promotion/credibility killer for obvious reasons. And what's the deal with flip flops? There must be thousands of styles of sandals that can transition easily into business casual without looking beach casual. A rule of thumb can be if it works at the beach it likely doesn't work in the office.
And don't wear a tie if it's called casual. I have colleagues who might just have to find scissors ...