My favorite alternate to wood flooring is Atlas Concorde's Etic porcelain tile and is very reasonably priced:
http://www.atlasconcorde.it/news.php?idarg=3&ida=52
Also, Armstrong makes a nice luxury vinyl tile in faux wood planks that I use in a lot of interior projects. Some look more realistic than others, so order samples!
http://www.armstrong.com/commflooringna/products/lvt/natural-creations-arborart/_/N-6uxZ1z141xv

See this blog post...they did a faux bois flooring over plywood. You could probably email her and ask her how comfortable it is walk on/how it held up to wear and tear etc.

http://justabouthome.wordpress.com/2011/08/01/faux-bois-floor-or-how-i-painted-a-fake-wood-floor/

I looked at the site Saramable pointed to and I would suggest that you use masking tape or something as guidance because her "planks" are pretty curvy and it is, IMHO, spoiling the result.

I know you said you don't want to paint hardwoods, but it seems like this would be one way to go about it:

1. put down a hardwood with an assertive grain to it, like hickory
2. stain it dark
3. put a thin coat of paint, or even a very light, opaque wood filler (the latter might be the most durable)
4. sand it to bring out the stain and grain
5. finish like normal.

I would get sample planks of your options and try some different techniques yourself.

I'd love to know as well. We wanted to do something like this with our stairs and ran into a brick wall trying to find info on how to do it. They're just your basic inexpensive red oak and we wouldn't want to keep them natural anyhow. They're still unfinished but we're considering a color chip floor coating like this. http://www.originalcolorchips.com/store/index.php?main_page=product_info&cPath=36_42&products_id=338

Thanks for all the suggestions! I've emailed the ladies at the site saramble recommended and checked out the LVT Armstrong faux wood plank flooring. I'd actually like to do something much bolder than the Armstrong—like the black and white version above perhaps—so painting options are still very welcome! Anyone have marine grade plywood flooring? Comfortable?

Plywood floors feel just about like hardwood underfoot - maybe a bit cushier. (I grew up in a partly unfinished house with a plywood-floored bedroom - we got to draw all over it, and it was awesome.) Make sure you get a nice enough grade that the faces are pretty smooth if that's the look you want.

HDF is another option - like MDF, except instead of medium, it's high density fiberboard.

Lonzebra sheet vinyl by Lonseal is almost the look you want - maybe a little small, but there is a black & white color.

The product isn't hand-painted faux bois -- it's a linoleum (I think) flooring product. Have no idea exactly which product, but I saw it in person in a kitchen design store in Europe.

With some perseverance and persistence (and a little luck), I think you should probably be able to find the exact product. The AT community is awesome in those kinds of searches!

Wow, linoleum would be ideal! Any suggestions much appreciated!

You could always hire the same guy ;)
http://www.franklinreport.com/Portfolio.aspx?pId=1000&Id=10658

I don't know where you live and what the rest of your house looks like etc, but what about concrete flooring?

You can actually buy tile that has this sort of look:

http://www.olympiatile.com/products/porcelain/glazed_porcelain/uonuon_glazed_collection.html

I would suggest this is your best option.

http://www.olympiatile.com/products/porcelain/glazed_porcelain/
uonuon_glazed_collection.html