It depends on what type of cork you're talking about. The gluedown kind, sure, I'd use that in a lightly used bath. It gets topcoated in place, and the glue that's used to put it down can be waterproof if you choose the non water based ones. The floating, no way. Even if they use HDF as the substrate to which the cork is glued, every single plank is a spot to let moisture through to both the subfloor and the plank composite structure itself. But, maybe you have little boy guests with better aim or their teen siblings who know that a shower curtain goes inside the tub? LOL! A guest bath can see some of the lightest use of any "wet" area in a home. On the other hand, it can see some of the worst abuse on an intermittent basis. I tend to be someone who plans for the worst case scenario rather than the best case one. I'd rather overbuild or overbuy, than have something fail or be inadequate. You've always struck me as much of the same mindset with your projects. I'm not sure how that observation tweaks your choices here, but I thought I'd throw it out there. :)

I don't know much about tiles, but with cork we've done hundreds of projects in the NY area with Cork flooring in the bathrooms/kitchens. Cork has no way of expanding or shrinking so it's a perfect product to put into wet areas. We work with Duro Design from Canada & we've never had a complaint in a very long time. http://howtochoosewoodflooring.com/ read this article it has a lot of useful information in it.
You can remove & replace cork just as easily as you would with flooring, actually easier.
You can have a floating cork option if you want & that would make things probably even better than to glue it down.
Let me know if you have any more questions- Steve

Here is a link that might be useful: Flooring

livewireoak & Steve, thanks for your replies. Livewire, you're right, I do usually build for earthquake/nuclear attack : ) but I am investigating surfaces other than porcelain tile because of the height differential between the hardwood hallway and the bathroom. There will be at least 1/2" differential because of the 1/2" plywood underlayment between the 2x6 planks and the Ditra (thinnest thing I can find as direct substrate). IF I could find something that minimized that height difference *and* be tough in a guest bath, it would be best.

Steve, I appreciate your professional input too. As I was reading about cork, I had thought that it *does* expand, but in all directions as opposed to only along the grain as hardwood does, which is why it makes good wine bottle stoppers, for instance. On another point, what I've read confirms your point about floating click vs. glue-down. I read that a waterbased surface sealer, e.g. Traffic, would be best to coat the seams, even if the cork tiles themselves are already treated, and to caulk the wall edges. What do you coat the seams with in wet areas? Thanks again for any further info you can share.

Floating Cork flooring is not recommended in Bathrooms. I have installed the gluedown cork in bathrooms. Check the manufacturer maintenance recommendations prior to sealing anything.