I just went to a beach wedding down in the Bahamas. That was tough to dress for. I had to ask the bride what was appropriate, since I figured a suit and dress shoes would be weird on the beach. Ended up wearing linen pants, sandals and a navy blazer with tie.
You're right, though, unless the wedding is somehow outside the norm in terms of location, theme etc., the guests should assume that a suit for men and dresses for women are the way to go.
Aha! You did something I'm pretty sure the assorted younger set isn't smart enough to do: ask one of us.