They are as proofed as a Sandisk card is but a lot slower (read the specs of the Sony card carefully!). Sandisk works fine for me.
About the weather sealing business. I use Goretex jackets to weather proof myself. The camera gets tucked under during a down pour.
I agree about the SD cards - as for jackets - if you set off on a day's walk in warm weather, then carrying a goretex jacket is a pain in the neck - too hot and uncomfortable - I'm already waterproof, so I don't need such extravagances, and I'm certainly not wearing one to tuck a camera under!
Without meaning to sound too flippant, I like wearing Norwegian Dale jackets (which are waterproof and very warm) but which don't have room for a camera underneath.
But the point here really is that if other camera manufacturer's 'environmental sealing' means you can carry it in a torrential downpour without worrying (and it does - at least for Nikon and Canon and Olympus and Pentax) - then it would be cool if Sony's 'environmental sealing' meant the same thing!