In my experience, CF cards definitely hold the edge on speed over SDHC cards, even class 10. CF cards currently reach 667x speeds, where as the fastest SD cards, class 10, only reach 66x speeds, as far as write speed goes. That would be 100mb/s for CF (or more commonly, 600x, which is 90mb/s), vs. 10mb/s for the fastest SD cards. When you see a speed rating on an SDHC card, beware! You may see 15mb/s or even 30mb/s SDHC cards, but that refers to read speed. I was unaware of this when I first started buying SD cards, so I have a variety of 15mb/s and 30mb/s read speed cards rated Class 2, 6, and 10. The difference in write speed amongst them is QUITE APPARENT, and nothing compared to CF write speeds. I have found the rating system for SD cards very misleading, and I've wasted a fair amount of money buying cards I though would give me 30mb/s write speed, when in fact they only offered 2 or 6 mb/s.
If you can pick up one of the really fast CF cards, they can often improve the characteristics of your camera as well. I recently moved up from using SanDisk 60mb/s CF cards to using Transcend 90mb/s CF cards (roughly the same price, but with Transcend you get just as much reliability but better speed.) I was surprised to find that the continuous frames on my 7D jumped from the default of 15 to about 23-24, and instead of simply halting once the limit was met, I could keep getting bursts of 2-3 shots with an extra second between...indefinitely.
It seems there are 1000x CF cards hitting the market these days from Lexar. They are pricy, but can supposedly reach speeds of 150mb/s. I would imagine you could see further benefits from one of those. I am not sure if those require UDMA-7 compatibility, and if they do, I would suspect they drop to 600x speed if you don't have that (as far as I know, all pro-grade Canon cameras do.)
I've also dropped a couple CF cards in the snow, however after letting them dry they have never had any problems. I've dropped a few SD cards in the snow a few times, and a couple of them stopped working after that (SanDisk brand, even! Not quite as reliable as the hearsay would indicate...)
I like the idea of using an SD card as a backup card, dual writing to the CF and SD (assuming that doesn't slow down write speed.) After a long day shooting nature, to the tune of a couple thousand shots, I always worry I'm going to lose one of my cards. It would bring a lot of peace of mind to have a backup of each.
