I think a whole cloth would be perfect. That's what brides from long ago made :)

I love the idea of a whole cloth if that is an option.
If not, here are a couple you could show her:

Here is a white DWR quilt....click through all 7 pics...pretty :D
http://www.amishcountrylanes.com/Pages/hs5002.shtml
Here is one that is white with a goldish accent you could show her just to see the depth with an extra color
http://www.amishcountrylanes.com/Pag...html?Patchwork
Goodluck! :D

How about many shades of off-white to taupe? I love these pics...:

A whole cloth quilt.

I also like the idea of tone on tone white and then do a pieced quilt in a design (of which I'm drawing a blank on an idea for).

Since she wants it all white/white you then may want to ask what quilted design she may want on it. If you put a rose pattern on it, or hearts, or something she doesn't really like....

I would also think about putting their names and wedding date on it either on the top as a design element or at least on the back in the label for it.

Good luck and let us know what you decide.

Ummmmm, everyone has their own defination of what white is...you should discuss it with her first...is it stark white, ivory, white with a bit of creamy color??? Rarely is stark white used unless as a blender. I would start there and then ask her preference of design as she obviously has her likes and dislikes and you are trying your best to please her. The ideas above are all wonderful but the best bet is talk with her. This would be a wonderful opportunity to learn about each other and have a nice experience together.

Maybe for starters you could make one beautiful all white pillow, with lots of lace and white embroidery or some such thing(perhaps a ring-bearer's pillow and keepsake for the bride) and check to see if she gets excited about that. I see too many posts on here from people who have knocked themselves out to make a special quilt for someone only to feel let down by the person's response. If they have no personal experience making something by hand, some people just don't get it. Maybe the bride is telling you that a quilt, as many non-quilters imagine them, is not something she hopes to own. Unless you had evidence that this soon-to-be DIL might really hope to have an all white handmade quilt, I think I'd check her gift registry for ideas and focus my quilting efforts on someone who will definitely appreciate them. It's much better than putting in a lot of effort only to receive a lukewarm response.

Thanks to all of you for the good suggestions. I really appreciate it! I'll let you know what I end up choosing.

Make her a solid white rag quilt I am thinking about makeing me one too. I think it will be georgous

A crazy quilt show casing your stitches with various white on white fabrics is beautiful. Good luck, I know they will love it.

If someone said that to me...I would pass out! It seems so daunting to do this...unless you have a long arm! For me...no can do.