I read an article a while back that stated they shipped Acropora frags from a collection site in the Caribbean (or Gulf of Mexico, can't remember exactly) and had a 100% survival rate when packed damp (wrapped in damp newspaper and then bagged) after 48 hours (I believe). I'll try and track down the article, but it seems completely plausible as many corals spend hours exposed to the sun and wind at low tide and so should be fine as long as they don't dry out or get cold.