Gah! That marble and stone is gorgeous. I vote to let that play center stage and go with a high quality flat or satin true white or complementary gray.

I totally agree w/ Berae. I'm a lover of wallpaper but I think it would be overkill in that bathroom.

I actually think it's nice as-is!

Consider a grass cloth or a nice textured paper in the granite room, no real pattern. I'd go for a nice color similar something you'd paint. A suede paint could also be a contrast to the stone.
Good luck and enjoy the process.

I was going to say grass / sisal type covering and see @delphblu beat me to it! So, on second look I am thinking instead of a contrast paint color why not match the stone and continue the color all the way up the wall. Then add some art with the same cherry wood framing to match the commode stand. Bring color in with towels.

I don't think you're crazy, particularly in the bathroom with the greenish tile, I think you have an opportunity to really play with the idea of natural elements- the rich green tile with light wood with crisp white... How about a bold, geometric print in rich, deep colors (earthy browns and greens and golds)? Or even a larger floral print, if it wasn't too fancy... then you could swap out the mirror for something larger that was framed with light wood (a statement piece) and maybe restain the sink console to be a lighter wood, and then use that same lighter stained wood for accessories, etc. around the room to lighten it a bit.

Go for it. Get some wallpaper you love. The tile placement mimics wainscotting.

I agree with @hippyvieja. That bathroom needs some interesting modern pattern. Although your tile/granite is lovely, it is a bit stark/minimal. If stark/minimal is your style goal, then leave it as it is. But if you like a bit of fun/opulence, there are many options available to you; wallpaper is certainly one. Because of the high moisture content, I would recommend a vinyl paper over a natural material like grasscloth which I love). And don't be afraid. Many of the new papers [like Graham & Browne's "Paste the Wall"] are easy to go up and easy to take down.

http://www.wallpaperfromthe70s.com/index.php?cat=c2_Floral.html

this is my favourite website for wallpaper. It is not prepasted -which is sometimes better, less mess.

For the marble bathroom, why not choose something from yesterday's AT post on grey wallpapers?

http://www.apartmenttherapy.com/ny/roundup/gorgeous-gray-wallpapers-roundup-128842

The great thing about bathrooms is they don't take much wallpaper or much work, so they are my favorite place to take risks.

http://www.troveline.com/wallpaper_detail_alula.php4
for the grey. I think it is botanical, but it looks like jellyfish seen from underwater -- a natural theme but not banal.

The dark green can be Asian in feel. I agree with tawny sisal or grass cloth.

I think you might also want to try bigger mirrors and more light.

Where I agree with others that the sisal or grass look would be a nice accent to the already perfectly finished stone, I'm finding it difficult to make sense of placing wallpaper in the bathroom... a place that contains much moisture. I'd be afraid of the paper falling over time.

If you really want a textured look, this can be achieved through simple painting techniques. Plus if you decide you don't like it later you can always paint over it. Wallpaper is a nightmare to take down.

I think paisley would work well in your marble bathroom... it could be an interesting play between traditional and modern, especially as you already have that going on with a traditional material -- a grey-veined white marble -- in a more modern application... streamlined combined with ornate.

Here are some options:

http://www.alluminare.com/design-your-own.asp?sku=F124-239;P;c71;26;15;51;5;200-9700-9700-14-nonex0-nonex0-0000-0-0#F124-239;P;c71;51;80;51;5;100-9700-9700-14-nonex0-nonex0-0000-0-0

http://blog.alluminare.com/paisley-on-the-walls/

http://www.designyourwall.com/store/Dazzle-Charcoal-Silver-floral-paisley-modern-wallpaper-AS16241d-pr-2592.html

Rapture, the one on the right:

http://www.harlequin.uk.com/collection.aspx?ID=15&P=6&C=

http://www.designyourwall.com/store/Dazzle-Lime-Brown-floral-paisley-modern-wallpaper-AS16261d-pr-2593.html

http://www.grahambrown.com/us/product/17782/Mystique%20:%20Green%20Wallpaper

As for the slate bathroom, there is so little wall area to cover with wallpaper, and it is so dark and heavy, I would either just brighten it with white paint or mirror the entire area (no seams except at corners).

Good luck!

If you purchase a wallpaper that is not pre pasted, you can use a specific glue specially designed for high moisture areas such as bathrooms.

How about a paint treatment instead? Like Ralph Lauren's faux suede? Or something with a subtle pattern instead of full blown wall paper?

I'm not typically a wall paper person but I'm starting to reconsider. I think you could definitely add a wallpaper to the bathrooms. If you concerned about the two competing too much I would try a similar color palate to the stone in a smaller graphic pattern. I would then let the mirror, light fixtures and any pictures accessories accent on the stone/wallpaper.

I think wallpaper in lavatories might work ok, but in full-on bathrooms, the steam might be a problem. (Never tried it, so I'm not certain.) I do know that every rental I ever lived in that had wallpaper in the bathroom -- at least half of the places I lived in -- the wallpaper was peeling in large or small ways. Which looks terrible. So if you do it, just make certain it's properly hung.

Getting wallpaper off the bathrooms' walls here was very difficult and unpleasant, and we never did remove all the adhesive under it. I've never had a good experience with wall paper, and never would bring it into my home.