I've been thinking about this too... my dress is HEAVY, but I fell in love with it so I don't care. Luckily it's got a good corset top to it, I'll probably get cups sewn in, and I'm just on the fence about spanx. Probably I'll do without. My future MIL told me I would "pass out in my dress it's so hot down there". Nice. She just wasn't crazy about Mexico. She's past it now, but I can't help wondering if she had a point. Hope that won't happen!
Related question: how do you stop from sweating too much??